GIFT   OF 
S.    W.    Kerns 


BIOLOGY 
LIBRARY 


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A    TEXT-BOOK    OF 
EXPERIMENTAL     PSYCHOLOGY 


A  TEXT-BOOK  OF 

EXPERIMENTAL 
PSYCHOLOGY 


BY 


CHARLES    S.     MYERS 

1 i 

LECTURER    IN   EXPERIMENTAL   PSYCHOLOGY 

IN    THE    UNIVERSITY   OF    CAMBRIDGE 

PROFESSOR   OF   PSYCHOLOGY   IN   KING'S  COLLEGE 

UNIVERSITY   OF    LONDON 


WITH   66   FIGURES  AND   DIAGRAMS 


NEW    YORK 

LONGMANS,    GREEN    &    CO. 

LONDON:   EDWARD   ARNOLD 

1909 

[All  rights  reserved] 


BIOLOGY 
LIBRARY 


PREFACE 


FOR  some  time  past  the  lack  of  a  Text-book  on  Experi- 
mental Psychology  has  been  keenly  felt.  The  literature  of 
the  subject  is  now  so  scattered  and  so  profuse,  that  a  student 
must  have  at  his  command  a  small  library  of  books  and 
periodicals  if  he  wishes  to  pursue  a  course  of  independent 
reading. 

In  endeavouring  to  supply  this  want,  I  do  not  attempt 
to  offer  a  "systematic"  Psychology.  On  the  contrary,  I 
assume  that  the  student  is  already  familiar  with  the 
elements  of  general  psychology.1  He  may  have  had  the 
opportunity  of  attending  an  introductory  course  of  lectures 
on  the  subject  which  were  accompanied  by  demonstrations, 
and  in  that  case  he  will  have  observed  how  artificial  is 
the  line  of  cleavage  between  general  and  experimental 
psychology. 

I  assume,  too,  that  he  does  not  approach  the  detailed 
study  of  experimental  psychology  in  ignorance  of  the 
general  structure  and  functions  of  the  nervous  system.  In 
the  following  pages  I  may  appear  at  times  to  have  laid 
undue  stress  on  purely  physiological  and  physical  considera- 

1  These  elements  will  be  found  clearly  and  concisely  presented  in 
Professor  G.  F.  Stout's  little  book,  The  Growidwork  of  Psychology 
(London,  1903). 

V 

M1145O6 


vi  PREFACE 

tions  in  their  relation  to  the  problems  of  experimental 
psychology.  But  the  ultimate  object,  which  has  influenced 
me  throughout,  has  been  to  describe  the  methods  and 
principles  of  psychological  experiment,  and  to  set  forth  the 
most  important  results  that  have  been  obtained  in  this 
field  of  research. 

Exigencies  of  space  have  compelled  me  to  omit  many 
topics  of  interest.  I  wish  I  could  have  included  a  chapter 
on  the  study  of  animal  behaviour,  and  could  have  dealt 
more  fully  with  experiments  upon  children  and  primitive 
peoples.  That  I  have  had  to  neglect  investigations  upon 
subconscious  and  abnormal  states  and  upon  the  mental 
effects  of  drugs,  causes  me  less  regret ;  for,  owing  to  unsatis- 
factory methods  and  insufficient  knowledge,  these  subjects 
are  as  yet  too  controversial  in  character  to  come  well  within 
the  scope  of  an  Elementary  Text-book. 

I  have  purposely  excluded  from  the  text  the  names  of 
all  workers,  save  in  a  discussion  of  their  views ;  the  names 
of  the  discoverers  of  facts  belong  to  the  history  of  the 
science,  rather  than  to  the  science  itself.  In  a  note, 
however,  at  the  end  of  each  chapter,  I  have  inserted  the 
titles  of  some  important  papers  (bibliographical  as  well  as 
experimental)  and  their  authors'  names,  which,  I  hope,  may 
guide  the  student's  further  reading. 

The  order  of  the  chapters  has  been  dictated  by  experi- 
ence in  teaching.  I  find  it  best  to  start  with  experimental 
work  on  sensation;  this,  on  the  whole,  gives  the  student 
less  difficulty,  alike  as  regards  manipulation  and  intro- 
spection. Owing  to  the  fulness  with  which  I  have  treated 
sensation,  the  account  of  experiments  relating  to  the  higher 
intellectual  processes  may  possibly  have  suffered.  I  feel 
very  strongly,  however,  that  the  best  training  for  the 


PREFACE  vii 

beginner  in  experimental  psychology  lies  in  the  field  of  sensa- 
tion. He  is  next  introduced  to  certain  statistical  methods, 
which  he  subsequently  applies  to  the  determination  of 
reaction  times.  Soon  after  follows  the  important  subject 
of  psycho-physical  methods,  a  careful  mastery  of  which  is 
essential  in  order  to  perform  the  quantitative  experiments, 
subsequently  described. 

It  was  only  after  much  hesitation  that  I  admitted  a 
series  of  practical  exercises  into  the  book.  Various  con- 
siderations deterred  me  from  so  doing, — for  example,  the 
knowledge  that  teachers  differ  as  to  the  details  of  experi- 
mental procedure  which  they  prefer  to  use  in  their  laboratory, 
and  my  experience  that  a  too  elaborately  written  account 
of  practical  work  encourages  thoughtlessness  and  scamping 
in  the  laboratory.  On  the  other  hand,  I  found  that  the 
addition  of  a  series  of  exercises  often  enabled  me  to  omit 
from  the  text  many  technical  and  other  details  which 
would  otherwise  have  embarrassed  the  argument.  I  can 
only  hope  that  I  have  given  sufficient  details  for  the 
exercises  to  prove  useful  to  the  intelligent  student,  who 
happens  to  be  working  without  an  instructor  at  his  call. 

I  hope,  too,  that  this  book  will  serve  to  spread  a 
more  exact  knowledge  of  the  scope  of  the  youthful 
science  of  experimental  psychology.  Even  educated  people 
in  this  country  often  confuse  experimental  psychology  with 
spiritualistic  research,  or  their  acquaintance  with  the 
subject  is  limited  to  a  hazy  notion  of  reaction  times.  There 
are  others  who  confuse  it  with  the  physiology  of  the  nervous 
system  and  of  the  sense  organs ;  and  others  again  who  just 
as  wrongly  style  it  the  "  new  "  psychology.  The  book  may 
also  serve  to  indicate  the  value  of  a  training  in  the  subject 
for  those  who  intend  to  devote  themselves  subsequently 


viii  PREFACE 

to   certain    other    studies,   more    especially   to    education, 
ethnology,  psychiatry,  or  aesthetics. 

I  regret  that  unforeseen  circumstances  have  delayed  its 
appearance,  and  I  owe  much  to  the  kind  forbearance  mean- 
while shown  to  me  by  my  publisher.  I  have  received  many 
invaluable  suggestions  from  Dr.  A.  S.  Lea,  Dr.  H.  K.  Ander- 
son, and  Dr.  R  K  Salaman,  who  have  most  kindly  read  my 
proofs.  Dr.  H.  Head  and  Mr.  Udny  Yule  have  also  helped 
me  in  Chapters  II.  and  X.  But  despite  the  vigilance  and 
aid  of  these  and  other  friends,  I  dare  not  hope  altogether 
to  have  escaped  the  many  errors  to  which  a  book,  that 
covers  so  much  new  ground  as  this,  is  liable. 

C.  S.  M. 

CAMBRIDGE, 

February,  1909. 


CONTENTS 


CHAPTER    I 

ON  THE  STANDPOINT  OF  EXPERIMENTAL  PSYCHOLOGY 

The  Relation  of  Experimental  to  General  Psychology — The  Conditions  of 
Experiment  in  Psychology— The  Response  of  the  Subject — The  Subject 
and  the  Experimenter — Introspection  in  Experiment — The  Behaviour 
of  the  Subject — Psychology  as  a  Science — Psychological  Abstraction — 
Experimental  in  relation  to  Physiological  Psychology — The  Aims  of 
Experimental  Psychology  .  .  .  .  pp.  1-10 

CHAPTER    II 

ON  CUTANEOUS  AND  VISCERAL  SENSATIONS 

Touch  Spots — Cold  Spots — Heat  Spots— The  Two  Systems  of  Cutaneous 
Sensibility— Pain  Spots— Pain  of  Non-cutaneous  Origin— The  Specific 
Nature  of  Pain  Sensations— Temperature  Adaptation— Touch  Adapta- 
tion— Other  Cutaneous  Experiences  .  .  "•  '-'•.-  pp.  11-19 

CHAPTER    III 
ON  AUDITORY  SENSATIONS 

The  Physical  Basis  of  Pitch  and  Loudness — The  Conduction  of  Sounds  to  the 
Inner  Ear — The  Conduction  of  Sounds  from  Ear  to  Ear — The  Physical 
Basis  of  Timbre — Fourier's  Theorem — Resonance — The  Middle  Ear — The 
Relations  of  Noise  to  Tone — Timbre — Tone  Nomenclature — The  Re- 
lation of  Overtones  to  Timbre — The  Threshold  of  Intensity — Auditory 
After-sensations  —  Tone  Character  —  The  Intensity  of  Simultaneous 
Tones— The  Range  in  Pitch  of  Audible  Tones— The  Smallest  Perceptible 
Difference  of  Pitch— Beats  and  Intertones  .  .  pp.  20-41 


x  CONTENTS 

CHAPTER    IV 

ON  AUDITORY  SENSATIONS  (concluded) 

Combination  Tones— Variation  Tones— Interruption  Tones— The  Relation 
of  Tones— The  Absolute  Determination  of  Pitch— The  Cochlea— 
Helmholtz's  Theory  of  Hearing — Extensions  of  Helmholtz's  Theory — 
Theories  of  Consonance — Criticism  of  Helmholtz's  Theory  of  Hearing 
—Rutherford's  Theory— Ewald's  Theory— Meyer's  Theory  pp.  42-62 

CHAPTER    V 

ON  LABYKINTHINE  AND  MOTOR  SENSATIONS 

The  Resemblance  of  these  Sensations.  Labyrinthine  Sensations  :  The  End 
Organs — Experimental  Interference  with  the  Canals — The  Mach- 
Breuer-Brown  Theory — Eye  Movements  produced  by  Rotation — Head 
Movement  in  Rotation — Giddiness — The  Utricle  and  Saccule.  Motor 
Sensations:  Their  Non-cutaneous  Origin — The  End  Organs — The 
Characters  of  Kinsesthesis — Illusions  of  Extent  of  Movement — Other 
Motor  Sensations — Other  Factors  determining  Extent  of  Movement — 
Awareness  of  Position — Comparison  between  the  Nervous  Connections 
of  the  Motor  and  Labyrinthine  Sensory  Apparatus  .  .  pp.  63-75 


CHAPTER    VI 

ON  VISUAL  SENSATIONS 

The  Characters  of  Visual  Sensations — The  Conditions  of  Colourless  Sensa- 
tions—Successive Contrast —  Simultaneous  Contrast  —  Simultaneous 
and  Successive  Induction — Spectral  Colour  Mixtures — Colour  Blindness 
— Flicker— Determinations  of  Brightness— The  Intrinsic  Light  of  the 
Retina — Purkinje's  Phenomenon— Rod  Vision— The  After-effects  of 
Momentary  Colour  Stimuli — The  Positive  After-image  .  pp.  76-91 


CHAPTER    VII 

ON  VISUAL  SENSATIONS  (concluded) 

The  Young-Helmholtz  Theory  of  Colour  Vision— Hering's  Theory  of  Colour 
Vision— Criticism  of  these  Theories— Contrast  in  a  Smoothly  Graded 
Disc— The  Nature  of  "Black"— Other  Theories— General  Criticism 

pp.  92-107 


CONTENTS  xi 

CHAPTER    VIII 

ON  GUSTATORY  AND  OLFACTORY  SENSATIONS 

Their  Close  Relation.  Gustatory  Sensations :  Simple  and  Complex  Tastes 
—The  Chemistry  of  Tasting  Substances— The  Region  of  Taste— The 
Action  of  Drugs — Compensation,  Rivalry,  and  Contrast.  Olfactory 
Sensations  :  The  Conditions  of  Smell — Classification  of  Smells — Anosmia 
— Fatigue — Compensation  and  Rivalry  .  .  .  pp.  108-116 

CHAPTER    IX 

ON  THE  SPECIFIC  ENERGY  OF  SENSATIONS 

Adequate  and  Inadequate  Stimuli — Effective  and  Ineffective  Stimuli — 
Specific  Nervous  Energy — Quality  and  Modality — Primary  Sensations 

pp.  117-122 

CHAPTER    X 

(  ON  STATISTICAL  METHODS 

Their  Importance— The  Mode— The  Mean— The  Mean  Variation— The 
Standard  Deviation  —  The  Coefficient  of  Variation  —  Significant 
Differences — The  Normal  Curve— The  Probable  Error— The  Median— 
The  Semi-interquartile  Range — Correlation  .  .  pp.  123-131 

CHAPTER    XI 

ON  REACTION  TIMES 

• 

Simple  and  Composite  Reactions — The  Reduced  Reaction  Time.  Simple 
Reactions  :  Sensorial  and  Muscular  Reactions — Psychological  Analysis 
of  Reaction  Times — Effects  of  Practice  and  Fatigue — Reaction  Move- 
ments— Natural  Reactions — Individual  Variations — Influence  of  Age 
and  Race — The  Personal  Equation— Other  Determinants  of  Reaction 
Times.  Composite  Reactions :  Recognitive  and  Discriminative  Reactions 
— Choice  Reactions — Naming  and  Reading  Reactions — Psychological 
Analysis  of  Composite  Reactions — Associative  Reactions — Forms  of 
Associative  Reactions — Association  Times — Mathematical  Analysis  of  Re- 
action Times— The  Physiological  Aspect  of  Reaction  Times  pp.  132-143 

CHAPTER  XII 

ON  MEMORY 

The  Perseverance  of  Experiences  —  Association  —  Memory  Images — The 
Kinds  of  Imagery — Memory  After  -  images  —  Experiments  on  the 


xii  CONTENTS 

Fading  of  Images — Comparison  without  Imagery — Influence  of  Time  and 
Speech  on  Imagery — The  Classification  of  Associations — Experiment  in 
Learning — The  Learning  and  Saving  Methods — The  Prompting  Method 
— The  Scoring  Method — Comparison  of  the  Learning  and  Scoring 
Methods — Other  Methods — Eelation  between  Scoring  Time  and  Associa- 
tion Strength — Influence  of  the  Length  of  the  Series — Influence  of  the 
Position  of  Syllables  and  of  Accent  and  Rhythm  .  .  pp.  144-161 


CHAPTER  XIII 

ON  MEMORY  (concluded) 

The  Rate  of  Forgetting — Retro-active  Inhibition — Retro-active  and  Remote 
Association  —  The  Behaviour  of  Related  Associations  —  Unconscious 
Association — Initial  and  Group  Reproduction — The  Independence  of 
Subsidiary  and  Principal  Associations  —  Mediate  Association  —  The 
Distribution  of  Repetitious — The  Influence  of  the  Age  of  Associations 
on  the  Results  of  Repetition — The  Influence  of  Time  on  Associations  of 
Different  Age— The  Most  Economical  Method  of  Learning— Improve- 
ment in  Mechanical  Learning — The  Influence  of  Speed  of  Reading — 
The  Learning  of  Sensible  Matter — The  Superiority  of  Rational  Learning 
—  The  Influence  of  Practice  and  Age  —  Individual  Differences  of 
Memory  .  .  .  .  ,  .  .  pp.  162-182 


CHAPTER   XIV 

ON  MUSCULAR  AND  MENTAL  WORK 
y 

The  Determination  of  Efficiency — The  Interrelation  of  Mental  and  Mus- 
cular Activity.  Muscular  Work  :  Ergography — Peripheral  and  Central 
Factors  in  Muscular  Fatigue — The  Inhibitory  Effect  of  Afferent  Im- 
pulses—The Different  Effects  of  Constant  and  Variable  Loads — The 
Sensory  Effect  of  Afferent  Impulses— The  Influence  of  Affection  and 
Interest  —  The  Influence  of  Mental  Fatigue  —  The  Complexity  of 
the  Conditions  of  Muscular  Efficiency.  Mental  Work  :  Methods  of 
Procedure— The  Interpolation  Method— The  ^Ssthesiometric  Test— The 
Ergographic  Test— The  Combination  Test— The  Letter-erasing  Test — 
The  Learning  Test— The  Calculation  Test— The  Relative  Reliability  of  the 
Methods— The  Continuous  Method— The  Mental  Work  Curve— Fatigue 
and  Practice — The  Factors  in  Fatigue — Spurts — Incitement — Adaptation 
—The  Effect  of  Rest  on  Work— The  Determination  of  Fatigability— The 
Determination  of  Improvability — The  Determination  of  Retentiveness 
of  Improvement— The  Meaning  of  the  Most  Favourable  Pause — Criti- 
cism of  Laboratory  Work  .....  pp.  183-200 


CONTENTS  xiii 


CHAPTER   XV 

ON  THE  PSYCHO-PHYSICAL  METHODS 

The  Uses  of  the  Methods — The  Method  of  Mean  Error :  The  Crude  Constant 
and  Crude  Average  Errors— The  Space  and  Time  Errors.  The  Limiting 
Method  :  Its  Simplest  Form — Procedure  by  Complete  Descent  and 
Ascent — The  Effects  of  Adaptation  and  Expectation— Other  Modifica- 
tions of  the  Method — The  Method  of  Serial  Groups.  The  Constant 
Method:  Reversals — Contrast  Effect— Side  Comparisons— "  Doubtful" 
and  "  Equal  "  Answers— The  Method  of  Equal-appearing  Intervals— 
The  Attitude  of  the  Subject— Correspondence  between  Results  of  Limit- 
ing and  Constant  Methods  .  ....  pp.  201-217 


CHAPTER   XVI 

ON  WEIGHT 

The  Effects  of  Tactual  and  Motor  Anaesthesia — Behaviour  in  Estimating 
Weight — Influence  of  the  Speed  of  Lifting  Weights — Tactual  and 
Visual  Influences — The  Size-Weight  Illusion — Motor  Attunement — The 
Possibility  of  Transference  of  Motor  Attunement — The  Comparison  of 
Motor  Attunement  with  the  Effects  of  Stimulating  the  Visual  Cortex 
— The  Comparison  of  Motor  Attunement  with  the  Association  of  Ideas 
—The  Sense  of  Effort— Discussion  of  two  Difficulties— Other  Difficul- 
ties .  .  .  .  .  .  .pp.  218-230 


CHAPTER   XVII 

ON  LOCAL  SIGNATURE 

Local  Sign — The  Cutaneous  Spatial  Threshold— Introspection  near  the 
Threshold — Variations  of  the  Spatial  Threshold — Cutaneous  Threshold 
for  Lines — The  Hibtological  Basis  of  the  Spatial  Threshold — Relative 
and  Absolute  Localisation  on  the  Skin — The  Basis  of  Cutaneous  and 
Retinal  Local  Signature — Moving  Retinal  Images— Retinal  After-sensa- 
tions of  Movement  .  .  .  .pp.  231-242 


CHAPTER   XVIII 

ON  SENSIBILITY  AND  SENSORY  ACUITY 

Discrimination  as  a  Factor  in   Sensation— The  Complexity  of  the  Condi- 
tions determining  Sensibility  —  Visual   Acuity  —  Visual  Efficiency  — 


xvi  CONTENTS 

EXERCISES  ON  CHAPTER   XI.  .  •  P-  370 

„    CHAPTERS  XII.  and  XIII.        .  p.  380 

,,    CHAPTER  XIV.  .  P-  383 

XV  -  P-  387 

),  55  IS  AY. 

XVI.  .  .  .  P.  388 

XVII P-  389 

XVIII P-  393 

XIX p.  398 

XX P-  400 

XXI P-  405 

XXII.  .  .  P-  408 

XXIII.  ...  P.  410 

XXIV.  .         •-.  •  P-  414 

XXV.  '.  P-  417 

INDEX  •  •   PP-  421-432 


ERRATA 

Page  117,  line  16,  for  "pressure"  read  "touch. 
Page  299,  line  9,  for  "shorter"  read  "longer." 


EXPERIMENTAL    PSYCHOLOGY 


CHAPTEE    I 

ON  THE  STANDPOINT   OF  EXPERIMENTAL 
PSYCHOLOGY l 

The  Relation  of  Experimental  to  General  Psychology. — 
Experimental  psychology  has  sometimes  been  styled  the 
"  new  "  or  "  scientific  "  psychology.  It  has  been  spoken 
of  as  if  it  were  quite  distinct  from,  and  independent  of,  the 
older  or  "  general "  psychology,  in  which  experiment  finds 
no  place.  Now  these  are  manifest  errors.  For  experiment 
in  psychology  is  at  least  as  old  as  Aristotle.  And  scientific 
work  is  possible  (e.g.  in  astronomy,  geology,  and  natural 
history)  under  conditions  which  preclude  experiment.  We 
must  regard  experimental  psychology  as  but  one  mode  of 
studying  psychological  problems,  not  all  of  which,  however, 
can  be  approached  from  the  side  of  experiment.  Far  from 
being  independent,  experimental  psychology  has  arisen 
as  a  refinement,  of  general  psychology.  Familiarity  with 
the  latter  is  essential  to  success  in  the  former. 

The  Conditions  of  Experiment  in  Psychology. — Experiment 
consists  in  observing  the  play  of  prescribed  conditions  ;  its 
object  is  to  secure  accurate  information.  So  long  as  the 
conditions  are  known  and  are  controllable,  an  experiment 

1  The  student  may  expect  to  understand  the  contents  of  this  chapter 
more  thoroughly  when  it  is  re-read  at  a  later  stage  of  his  progress. 
I 


PSYCHOLOGY 


.can.  he.mpeated  by  tliQ-game  or  by  other  investigators,  the 
fcngip^l'joBs^fv^tien^aii.'be  confirmed  or  modified,  and  the 
experiment  can  be  made  to  yield  further  information  by 
simplification  or  complication  of  these  conditions. 

Experimental  psychology  studies  the  responses  of 
individuals  to  prescribed  conditions.  Not  every  response, 
however,  possesses  psychological  interest.  Experimental 
psychology  is  directly  concerned  only  with  those  responses 
which  throw  light  on  the  analysis  and  the  course  of  mental 
states. 

The  conditions  in  psychological  experiment  are  the 
internal  conditions  of  the  individual  (or  subject)  on  the  one 
hand,  and  the  conditions  of  his  environment  on  the  other. 
A  psychological  experiment  may  accordingly  be  modified 
by  altering  either  the  mental  attitude  of  the  subject  or  the 
outer  influences  to  which  he  is  exposed. 

The  Response  of  the  Subject.  —  The  subject  responds  to  a 
psychological  experiment  by  undergoing  changes  in  inward 
experience  or  in  outward  action,  usually  in  both  ways.  It 
is  clear  that  the  former  mode  of  response  can  only  be 
studied  by  the  subject  himself  ;  his  states  of  consciousness, 
his  experience  throughout  the  experiment,  can  be  revealed 
solely  by  his  own  introspection.  On  the  other  hand,  his 
outward  action,  his  behaviour  towards  the  experiment,  is 
best  studied  by  an  independent  observer.  For,  in  the  first 
place,  it  is  generally  admitted  that  no  man  is  a  judge  of  his 
own  actions  ;  and,  secondly,  the  subject's  attention  during 
a  psychological  experiment  is,  as  a  rule,  already  fully  occu- 
pied in  introspection.  Therefore,  in  all  but  the  simplest 
psychological  investigations,  the  co-operation  of  two  persons 
is  desirable,  —  the  subject  recording  his  inner  experiences, 
and  the  experimenter  recording  the  subject's  outward 
behaviour. 

The  Subject  and  the  Experimenter.  —  If  the  same  indi- 
vidual be  at  once  subject  and  experimenter,  he  must  needs 
prescribe  for  himself  the  experimental  conditions,  and  is 


ITS  STANDPOINT  3 

thus  in  the  position  to  observe  and  to  appreciate  the  results 
obtained.  He  is,  we  may  say,  "  fully  informed."  He  knows 
what  is  about  to  happen,  and  he  knows  precisely  what  to 
look  for.  Under  such  conditions,  auto-suggestion  has  full 
play  with  him :  "  the  wish  is  father  to  the  thought."  He 
knows,  too,  whether  he  has  succeeded  or  failed  in  the  object 
of  his  experiment,  and  is  encouraged  or  depressed  accord- 
ingly. 

On  the  other  hand,  if  an  experimenter  co-operates  he 
can  arrange  the  experimental  conditions  so  that  the  subject 
is  more  or  less  "  uninformed,"  a  more  complete  freedom 
from  prejudice  being  thus  attained.  Then,  also,  the  experi- 
menter may  purposely  repeat  the  same  experiment  when 
the  subject  is  in  different  stages  of  foreknowledge,  practice, 
or  fatigue ;  or  he  may  perform  it  under  like  conditions 
upon  different  subjects. 

Introspection  in  Experiment. — It  is  frequently  urged 
that  the  act  of  introspection  cannot  fail  to  disturb  the 
normal  course  of  the  subject's  attention.  An  endeavour  is 
as  often  made  to  evade  this  objection  by  the  substitution 
of  retrospection  for  introspection  ;  which  in  turn  prompts  the 
further  objection  that  memory  cannot  be  relied  upon  to  give 
a  psychologically  just  description  of  a  bygone  experience. 
But  the  use  of  the  experimental  method  reduces,  even 
though  it  does  not  abolish,  the  force  of  each  of  these 
objections.  With  increasing  practice,  the  attention  can  be 
trained  to  oscillate  rapidly  to  and  fro,  the  subject  now 
responding  to  experimental  conditions,  now  observing  the 
contents  of  consciousness  during  his  response  ;  just  as  with 
practice  he  can  successfully  dictate  a  letter  and  read  a 
book,  to  all  outward  appearances  simultaneously. 

It  may  reasonably  be  objected  that  the  repetition  of  an 
experiment  can  never  bring  with  it  an  exact  repetition  of 
the  subject's  original  mental  state.  On  the  other  hand, 
practice  enables  him  to  detect  experiences  which  had  pre- 
viously escaped  him,  and  generally  to  improve  his  memory 


4  EXPERIMENTAL  PSYCHOLOGY 

for  bygone  states  of  consciousness.     That  is  to  say,  practice 
improves  his  power  of  introspection  and  retrospection. 

Fundamentally,  of  course,  all  introspection  is  retrospec- 
tion, and  the  objection  to  either  arises,  in  great  part, 
from  the  mistaken  notion  that  we  can  ever  describe 
momentary  states  of  consciousness.  It  has  been  truly  said 
that  "  neither  my  experience  as  a  whole,  nor  the  position 
nor  relations  of  any  part  within  that  whole,  can  be  given  as 
the  content  of  momentary  consciousness.  The  momentary 
consciousness  is  only  one  link  in  the  series  which  constitutes 
my  experience." 

The  limits  to  the  scope  of  introspection,  on  which  stress 
is  laid  in  works  dealing  with  general  psychology,  are  of 
course  equally  valid  in  the  region  of  experimental  psycho- 
logy. No  one  can  adequately  perform  introspection,  when 
dominated  by  intense  passion  or  by  intense  desire ;  nor  can 
he  adequately  describe  such  experiences,  when  the  passion 
or  the  desire  has  passed  away.  Attention  to  the  pleasure 
or  pain  of  an  experience  inevitably  modifies  that  pleasure 
or  pain.  The  more  prominently  affective  or  conative  be  a 
state  of  consciousness,  the  more  difficult  it  is  to  study  it. 
In  such  instances,  especially,  we  may  hope  to  advance 
our  knowledge  by  observing  the  subject's  outward  action  or 
behaviour. 

The  Behaviour  of  the  Subject. — The  study  of  the  subject's 
behaviour  always  forms  an  important  part  of  psychological 
investigation.  By  means  of  it,  we  are  enabled  to  obtain 
numerical  data  which  serve  as  an  index  of  mental  activity 
or  sensibility ;  we  are  enabled  to  observe  variations  in  the 
accuracy  or  in  the  mode  of  the  response,  which,  aided  by 
introspection,  throw  light  on  the  nature  of  the  conscious 
processes  involved,  or  reveal  differences  in  different  indi- 
viduals ;  we  are  enabled  to  record  involuntary  movements, 
e.g.  movements  of  the  limbs  and  changes  in  the  circulation 
or  respiration. 

Such  data  clearly  gain  in  significance  when  they  can  be 


ITS  STANDPOINT  5 

correlated  with  and  confirmed  by  the  subject's  introspective 
record.  Accordingly,  it  is  a  golden  rule  that  introspection 
should  never  be  omitted  in  a  psychological  'experiment. 
There  are,  however,  conditions  under  which  it  is  impossible 
to  avail  ourselves  of  the  aid  of  introspection,  as  in  the 
investigation  of  unconscious  processes,  in  experiments  on 
animals  and  sometimes  in  experiments  on  children  or 
savages,  or  on  individuals  in  abnormal  (e.g.  hypnotic  or 
pathological)  conditions.  In  all  circumstances  the  dangers 
of  directly  deducing  the  mental  state  of  an  individual 
from  observation  of  his  behaviour  cannot  be  too  strongly 
emphasised. 

Psychology  as  a  Science. — We  are  now  in  a  position  to 
realise  that  it  is  only  the  possibility  of  giving  a  physical 
expression  to  mental  states  which  confers  on  general  and 
experimental  psychology  the  rank  of  a  Science.  This  physical 
expression  is  obtained  in  two  ways, — first  by  the  observation 
of  the  subject's  outward  behaviour,  and  secondly  by  the 
description  of  the  subject's  inner  experience.  About  the 
former  we  need  say  nothing  more  at  present;  so  clearly 
is  outward  behaviour  a  mode  of  physical  expression.  But 
it  may  at  first  sight  seem  strange  to  say  that  when  the 
subject  describes  his  own  mental  states,  he  is  again  giving 
vent  to  physical  expression.  Yet  such  is  really  the  case ; 
otherwise  a  Science  of  psychology  would  be  impossible. 
For  from  the  psychological  standpoint,  as  we  have  seen, 
no  one  can  observe  the  mental  states  of  another.  Mental 
states  are  their  subject's  private  property, — a  contrast  to 
the  common  property  of  objects  of  the  physical  world.  As 
soon,  however,  as  a  subject  takes  the  trouble  to  record  his 
mental  states,  he  expresses  them  physically.  He  speaks  or 
he  writes, — that  is  to  say,  he  employs  physical  movements 
which  are  patent  to  and  significant  for  his  fellow  men. 

It  is  the  object  of  experimental  psychology,  as  of  all 
other  experimental  sciences,  to  describe  the  complex  in 
terms  of  the  simple.  Just  as  physics  attempts  to  express 


6  EXPERIMENTAL  PSYCHOLOGY 

objective  experience,  so  experimental  psychology  attempts 
to  express  subjective  experience  as  a  series  of  equations, 
reducing  the  complex  on  the  one  side  of  the  equation  to 
its  elementary  components  on  the  other  side. 

From  one  aspect  a  certain  mixture  of  hydrogen  and 
oxygen  is  identical  with  an  equal  mass  of  water.  From  the 
same  standpoint  the  binocular  presentation  of  two  stereo- 
scopic views  may  be  considered  as  identical  with  the 
single  view  in  relief  which  it  yields;  or  the  simultaneous 
presentation  of  a  tone  with  its  overtones  may  be  considered 
as  identical  with  the  peculiar  timbre  which  results.  But 
neither  in  chemistry  nor  in  psychology  are  we  satisfied 
with  equations  that  have  a  merely  existential  import. 
Although  the  hydrogen  and  oxygen  remain  undestroyed 
during  their  transformation  into  water,  we  cannot  overlook 
the  fact  that  important  alterations  have  taken  place  in  their 
relations  to  one  another, — that  they  have  fused  to  form 
a  chemical  compound  instead  of  being,  as  previously,  a 
mechanical  mixture.  So,  too,  in  our  psychological  examples, 
we  cannot  overlook  the  fact  that  the  several  tonal  sensations 
have  fused  to  create  a  totally  new  experience  of  timbre,  or 
that  the  two  visual  perceptions  have  fused  to  create  a  totally 
new  experience  of  relief. 

Both  chemistry  and  psychology  must  recognise  the 
inexplicable  nature  of  this  fusion.  The  former  may  attempt 
to  reduce  the  characteristic  properties  of  water  to  terms  of 
altered  molecular  composition  and  movement.  Such  so- 
called  explanation,  however,  consists  merely  in  describing 
the  phenomena  in  other  language,  in  translating  them  into 
other  modes  of  experience.  The  conditions  or  equivalents 
of  fusion  are  not  its  explanation ;  its  esse  is  its  per  dpi. 

But  this  apparent  similarity  of  psychical  to  chemical 
fusion  breaks  down  on  closer  inspection.  In  the  first  place, 
neither  the  sensations  nor  the  results  of  the  fusion  could 
ever  be  experienced,  were  it  not  that  they  go  to  form  part 
of,  and  to  fuse  with,  the  subject  whose  experience  they  are. 


ITS  STANDPOINT  7 

In  the  second  place,  the  very  experience  to  which  they, 
each  or  together,  give  rise,  is  determined  by  the  past 
experiences  and  by  the  present  condition  of  the  subject. 
The  experimental  analysis  and  synthesis  of  the  subject's 
experiences  must  therefore  be  supplemented  by  the  study 
of  the  personality  of  the  subject, — a  field  in  which  patho- 
logical and  hypnotic  investigations  promise  a  rich  harvest, 
but  which  lies  in  great  part  beyond  the  present  scope  of 
experimental  psychology. 

Lastly,  let  us  remember  that  we  are  quite  unconscious  of 
any  fusion  between  two  (or  more)  simultaneous  sensations 
or  perceptions,  in  the  examples  above  chosen.  To  be 
convinced  of  this,  we  have  only  to  look  into  a  stereoscope 
or  to  listen  to  the  tone  of  a  musical  instrument.  The 
complex,  i.e.  the  relief  or  timbre,  is  all  that  we  are  aware 
of.  In  our  ignorance  we  deem  it  simple;  the  different 
experiences  of  the  two  eyes,  or  the  presence  and  inter- 
relation of  overtones,  are  only  brought  to  our  notice  by 
special  methods.  Thus,  without  pursuing  the  subject 
further,  we  see  that  it  is  psychologically  untrue  to  say 
that  we  first  have  sensation  (H2)  and  its  companion  sensation 
(0),  which  then,  by  the  touch  of  a  fairy  wand,  suddenly 
become  transformed  into  a  new  complex  (H20).  All  we 
can  truly  say  is  that  stimuli,  which  separately  give  rise  to 
unlike  experiences,  may,  when  acting  together,  give  rise  to 
a  totally  different  complex,  without  evoking  (or,  to  speak 
more  generally,  without  necessarily  evoking)  the  unlike 
experiences  themselves. 

Psychological  Abstraction. — We  shall  begin  the  study  of 
experimental  psychology  by  considering  the  most  elementary 
mental  units  into  which  we  can  analyse  the  presentations' 
of  external  objects, — sensations.  It  might  be  thought  that 
any  chance  stimulation  of  the  sensory  end  organs  of  our 
body  must  inevitably  yield  a  sensation.  But,  as  we  have 
just  pointed  out,  under  no  circumstances  are  our  sensory 
experiences  isolated  independent  parts  of  our  mental  system. 


8  EXPERIMENTAL  PSYCHOLOGY 

They  form  with  one  another  and  with  the  rest  of  our 
mental  system  complexes  which  have  been  evolved  for 
the  express  purpose  of  securing  adjustment  to  external 
surroundings. 

In  our  endeavour  to  obtain  sensations  in  a  state  of 
requisite  purity,  we  have  often  to  adopt  special  experi- 
mental and  introspective  measures,  stripping  presentations, 
so  far  as  possible,  of  all  those  characters  which  ordinarily 
make  them  vehicles  of  meaning.  In  the  course  of  such 
processes  of  abstraction,  we  shall  at  times  discover  and 
study  sensations,  of  whose  nature  we  were  previously 
scarcely  aware,  either  owing  to  their  invariable  coexistence 
with  other  mental  states,  or  owing  to  their  relative  un- 
importance as  a  means  of  interpreting,  or  of  consciously 
adjusting  ourselves  to,  the  outer  world. 

It  is  commonly  supposed  that  in  the  developing  in- 
dividual these  simple  mental  states  form  the  primary 
substratum  from  which  his  more  complex  states  are 
subsequently  developed.  An  exactly  opposite  view  is 
nearer  the  truth.  The  clearer,  simpler  states  should  be 
broadly  regarded  as  secondary  to  vaguer,  more  complex 
state  from  which  they  are  derived  through  the  analytic, 
differentiating  activity  of  the  growing  mind. 

Experimental  in  relation  to  Physiological  Psychology. — In 
the  study  of  sensations,  the  experimental  psychologist — who 
investigates  mental  states — proceeds  hand  in  hand  with  the 
physiologist, — the  investigator  of  the  functions  of  living 
matter.  Here  experimental  psychology  and  physiological 
psychology  are  inseparable  for  a  thorough  treatment  of  the 
subject,  protoplasmic  activity  throwing  light  on  the  ultimate 
analysis  of  sensation,  and  sensation  throwing  light  on  the 
significance  of  protoplasmic  activity,  as  our  knowledge  of 
each  progresses. 

In  other  regions  of  psychological  investigation,  the 
connection  between  experimental  and  physiological  psy- 
chology is  not  so  close.  For  example,  by  far  the  most 


ITS  STANDPOINT  9 

important  discoveries  made  by  experimental  psychology 
in  regard  to  memory,  comparison,  and  mental  work 
are  at  present  quite  devoid  of  physiological  basis.  It  is 
important  early  to  recognise  how  independent  the  truths 
of  experimental  psychology  are  of  the  determination  of 
the  corresponding  neural  processes  by  physiological  psy- 
chology. 

Some  psychologists,  indeed,  refuse  to  accept  psycho- 
physical  parallelism  as  a  principle  applicable  to  all  mental 
processes.  But  provided  that  a  proper  meaning  be  attached 
to  the  term  "  psycho-physical,"  a  thoroughgoing  parallelism 
probably  affords  the  best  working  hypothesis  for  experi- 
mental psychology. 

"  Physical "  phenomena  are  the  result  of  purely 
mechanical  conditions.  If  those  conditions  are  known,  the 
result  can  be  predicted.  It  is,  however,  only  in  com- 
paratively simple,  and  usually  in  artificially  established, 
conditions  that  the  physiologist  can  accurately  predict  what 
reaction  will  occur  with  a  given  stimulus.  The  living  body 
is  characterised  by  unknown  "  vital "  activities  as  well  as  by 
known  "mechanical"  activities.  There  are  many  who 
believe  that  the  two  differ  from  one  another  rather  in 
degree  and  complexity  than  in  kind;  for  the  history  of 
physiology  shows  how  activities  which  had  been  con- 
sidered as  vital  by  one  generation,  have  been  resolved  into 
mechanical  activities  by  another  generation  of  physiologists. 
Yet  the  fact  remains, — and  it  applies  especially  to  the 
nervous  system  of  the  intact  animal, — the  conditions  are 
so  complex  and  obscure  that  there  are  many  physiological 
results  which  it  is  impossible  to  predict. 

Similarly  in  psychology  some  experiences  occur  in  a 
purely,  or  almost  purely,  mechanical  manner ;  the  con- 
ditions are  so  well  known  that  a  definite  result  may  with 
fair  confidence  be  predicted.  But,  as  in  the  physiology  of  the 
nervous  system,  "  mechanism  "  has  the  strictest  limitations. 
There  is  thus  a  true  "  psycho-physiological "  correlation,  and 


to  EXPERIMENTAL  PSYCHOLOGY 

it  is  in  this  sense  that  the  term  "  psycho-physical ;>  parallel- 
ism must  be  understood. 

The  Aims  of  Experimental  Psychology. — The  difficulty  of 
prediction  to  which  attention  has  just  been  drawn,  is  often 
used  to  support  the  argument  that  a  Science  of  experi- 
mental psychology  is  impossible.  It  is  urged  that  a  given 
individual  varies  at  different  times,  and  that  individuals 
differ  among  themselves  so  greatly  as  to  preclude  the 
possibility  of  generalisation.  But  experimental  psychology 
is  not  engaged  merely  with  general  problems,  e.g.  studying 
thresholds,  determining  the  scope  of  attention,  or  fixing 
the  limits  of  memory.  It  has  also,  as  we  shall  see  later,  to 
determine  how  such  "  properties  "  of  the  mind  are  affected 
in  any  given  individual  by  different  conditions,  and  how  far 
and  for  what  reason  they  are  different  in  different  individuals. 
The  difficulties  of  prediction,  therefore,  enhance  rather  than 
detract  from  the  scientific  interest  of  the  subject.  Similar 
difficulties  of  lower  or  higher  order  thwart  our  prediction  of 
the  weather  or  of  the  course  of  evolution:  where  also  the 
conditions  are  too  complex  and  too  changeable  for  us  to 
foretell  the  certainty  and  order  of  events.  It  is  the  aim  of 
all  Science,  and  hence  the  aim  of  experimental  psychology, 
to  analyse,  so  far  as  possible,  the  conditions  which  may  be 
at  work,  and  to  determine  the  results  which  must  follow, 
provided  that  those  conditions  are  present. 


CHAPTEE  II 
ON   CUTANEOUS   AND  VISCERAL  SENSATIONS 

THE  exploration  of  the  skin  by  punctate  stimuli  (exps.  1-7) 
shows  that  its  sensibility  to  touch,  pain,  cold,  and  heat  is 
not  distributed  uniformly  over  the  surface.  The  skin  con- 
tains certain  "  spots  "  which  are  particularly  sensitive  to  the 
lightest  touch,  others  which  are  sensitive  to  pain,  others 
again  which  are  sensitive  to  cold,  and  others  to  heat. 

Touch  Spots. — On  hairy  parts  of  the  skin,  a  touch  spot  is 
to  be  found  over  the  site  of  each  hair  root  or  follicle  (exp.  4). 
A  few  touch  spots  are  also  met  with  between  the  hairs  ; 
they  abound  on  the  hairless  surface  of  the  palm  and  sole. 
A  rich  plexus  of  nerve  fibres  surrounds  each  hair  follicle, 
the  latter  being  the  probable  seat  of  the  tactile  end  organ. 
It  has  been  suggested  that,  on  hairless  surfaces,  Meissner's 
corpuscles  are  the  end  organs  corresponding  to  those  of  the 
hair  follicles.  At  the  tip  of  the  finger,  touch  spots  are  so 
abundant  as  to  be  inseparable ;  and  here  the  number  of 
Meissner's  corpuscles  is  correspondingly  large.  Touch  spots 
are  absent  on  the  glans  penis  and,  according  to  certain 
observers,  on  the  cornea.  They  react  to  stimuli  which  are 
far  too  weak  to  excite  nerve  fibres  directly.  They  react 
not  only  to  pressure,  but  also  to  traction  of  the  skin,  i.e.  to 
pull  as  well  as  to  push ;  the  sensation  being  the  same  for 
either  form  of  stimulus.  The  skin  is  sensitive  to  diffuse 
light  touch  (e.g.  to  the  touch  of  cotton  wool)  where  punctate 
exploration  fails  to  show  the  existence  of  touch  spots. 

Cold   Spots. — They   are   for   the   most    part  irregularly 


12  EXPERIMENTAL  PSYCHOLOGY 

grouped,  sometimes  forming  chains  or  clusters,  but  also 
occurring  as  isolated  spots.  It  has  been  suggested  that 
they  correspond  in  distribution  with  the  end  bulbs  in  the 
skin.  Individual  cold  spots  vary  considerably  in  sensitivity 
(exp.  1). 

Heat  Spots. — These  are  less  numerous,  and  react  more 
slowly  than  the  cold  spots  (exp.  2). 

The  sensations  derived  from  cold  spots  and  heat  spots, 
especially  from  the  latter,  are  more  diffuse,  less  definitely 
localisable,  than  those  afforded  by  touch  spots.  Menthol  is 
believed  to  produce  its  characteristic  effect,  by  causing 
hypersesthesia  of  the  cold  spots.  Carbonic  acid  gas  is  said 
to  cause  a  similar  hypersesthesia  of  heat  spots. 

The  Two  Systems  of  Cutaneous  Sensibility. — But  our 
sensations  of  pressure  and  temperature  cannot  wholly  be 
accounted  for  by  the  reactions  of  the  touch,  heat,  and  cold 
spots  of  the  skin.  For  when  all  the  nerve  fibres  supplying 
an  area  of  the  skin  have  been  divided,  sensibility  to 
temperature,  to  light  touch  (e.g.  to  the  touch  of  cotton  wool), 
and  to  cutaneous  pain  is  immediately  lost ;  but  sensibility 
to  heavier  touch  (e.g.  to  the  touch  of  a  pin's  head),  and  to 
deep-seated  pain  over  the  same  area  nevertheless  remains. 
These  residual  sensations  must  be  due  to  the  preservation 
and  excitation  of  structures  underlying  the  skin,  situated 
presumably  in  or  around  tendons  and  muscles  the  nerves  of 
which  are  known  to  contain  sensory  fibres. 

It  would  seem,  too,  that  certain  other  cutaneous  sensations 
are  likewise  of  double  origin;  that,  in  addition  to  the 
apparatus  for  "  heat "  and  for  "  cold,"  demonstrated  by 
punctate  exploration  of  the  skin,  there  is  another  non- 
punctate  system  in  the  skin  concerned  in  the  development 
of  sensations  of  "  warmth  "  and  "  coolness."  More  precisely, 
it  appears  that  while  the  response  to  superficially  painful 
and  to  hot  and  cold  stimuli  is  the  functional  expression  of 
one  system  of  cutaneous  sensibility,  the  appreciation  of  light 
touch,  warmth,  and  coolness,  and  the  power  of  precise 


CUTANEOUS  AND  VISCERAL  SENSATIONS     13 

cutaneous  localisation  are  the  expression  of  another  system 
of  cutaneous  sensibility.  For,  after  injury  to  peripheral 
nerve  fibres,  stages  occur  during  recovery  from  which  one 
of  these  two  systems  is  absent,  while  the  other  remains. 

Thus,  during  recovery  from  the  effects  of  section  of  a 
cutaneous  sensory  nerve,  a  stage  has  been  observed  in  which 
heat,  cold,  and  pain,  corresponding  to  the  heat,  cold,  and 
pain  spots,  are  felt ;  while  sensations  of  warmth,  coolness, 
and  light  touch,  and  the  ability  to  distinguish  two  neigh- 
bouring touches  from  one  another,  are  wanting.  Whereas 
over  the  normal  skin  the  heat  spots  and  the  cold  spots 
are  set  in  an  area  sensitive  to  cool  and  warm  stimuli,  the 
result  of  nerve  section  is  to  produce  a  state  in  which  only 
the  heat  and  cold  spots  are  present ;  that  is  to  say,  a  state 
in  which  stimuli,  having  a  temperature  between  about  26° 
and  37°  C.,  produce  no  thermal  effect.  Below  or  above 
these  limits,  the  heat  or  the  cold  spots  react  explosively, 
yielding  characteristically  diffuse  and  tingling  sensations, 
the  intensity  of  which  is  apparently  independent  of  the 
degree  of  heat  or  cold,  so  long  as  the  stimulus  employed  is 
at  all  adequate. 

But  this  is  not  all.  While,  after  nerve  section,  the 
greater  part  of  the  affected  cutaneous  area  shows  the  above 
"  protopathic  "  state  of  sensibility,  small  outlying  cutaneous 
areas  may  at  the  same  time  be  found  in  which  sensations 
of  heat,  cold,  and  pain  are  absent,  while  the  diffused  sensi- 
bility to  light  touch,  to  warmth,  and  to  coolness  yet  remains. 
The  prick  of  a  pin,  under  these  conditions,  is  felt  merely  as 
a  sensation  of  pointedness ;  the  sense  of  pain  is  gone,  but 
acmsesthesia l  remains. 

Under  other  abnormal  conditions,  this  latter,  "  epicritic," 
system  of  sensibility  may  prove  to  be  the  only  one  present 
over  more  extensive  cutaneous  areas.  In  the  normal 
viscera  there  is  some  evidence  that  it  is  altogether  wanting, 

1  Dr.  Head  suggests  this  word  as  preferable  to  the  more  generally  used 
"  acusesthesia." 


14  EXPERIMENTAL  PSYCHOLOGY 

the  protopathic  system  being  alone  present.  The  proto- 
pathic  appears  to  be  more  primitive  than  the  epicritic.  It 
is  characterised  by  imperfect  power  of  localisation.  It  is 
less  liable  to  disappear  and  is  readier  to  reappear  than  the 
epicritic  system. 

According  to  Head  and  his  collaborators,  to  whose  work 
these  recent  additions  to  our  knowledge  are  due,  these  two 
systems  of  cutaneous  sensibility  compel  us  to  assume  the 
existence  of  two  differently  distributed  systems  of  peri- 
pheral nerve  fibres.  It  would  lead  us  too  far  afield  to 
discuss  the  cogency  of  this  assumption.  But  at  all  events 
it  is  important  to  remember  that,  when  once  they  have 
reached  the  spinal  cord,  the  impulses  are  found  arranged  in 
quite  a  different  manner.  All  thermal  impulses  are  now 
grouped  together,  irrespectively  of  the  system  in  which 
those  impulses  arise,  and  a  similar  grouping  occurs  in  regard 
to  the  impulses  of  touch  and  pain.  Further,  as  the  im- 
pulses ascend  in  the  cord,  they  cross  close  by  the  central 
canal  to  the  opposite  side,  but  the  rapidity  with  which 
they  cross  in  their  ascent  varies  with  the  kind  of  impulse 
they  conduct.  In  the  case  of  tactual  impulses,  the  crossing 
is  much  less  rapid  than  in  the  case  of  painful  and  thermal 
impulses.  From  this  brief  account  it  is  sufficiently  obvious 
that  the  effects,  which  localised  injuries  to  different  areas  of 
the  cord  produce  upon  sensibility  to  temperature,  pressure, 
and  pain,  must  differ  profoundly  from  the  (immediate  or 
remote)  effects  of  the  section  of  peripheral  sensory  nerve 
fibres. 

Pain  Spots. — These  are,  on  the  whole,  far  more  abundant 
than  the  heat,  cold,  or  touch  spots,  and  are  always 
difficult  to  demonstrate  (exps.  5,  6).  They  are  present  in 
the  cornea,  where  touch  spots  are  said  to  be  absent.  On 
the  other  hand,  the  sensation  of  pain  is  altogether  absent 
over  an  area  of  the  mucous  membrane  of  the  mouth  corre- 
sponding to  part  of  the  cheek  (exp.  8). 

There  are  great  individual  differences  in  the  sensitivity 


CUTANEOUS  AND  VISCERAL  SENSATIONS     15 

of  pain  spots.  Indeed,  the  number  of  pain  spots  that 
can  be  found  within  any  given  area  depends  on  the  strength 
of  the  stimulus  employed.  Herein  they  present  a  striking 
contrast  to  touch  spots,  which  are  much  fewer  and  show 
little  variability  in  threshold.  Pain  spots  are  always  far 
less  sensitive  than  touch  spots.  Nevertheless,  they  give 
rise  to  pain  sensations,  when  the  stimulus  is  too  weak  to 
excite  the  nerve  endings  directly. 

Sensations  of  pain  are  more  diffuse  and  difficult  to 
localise,  and,  especially  when  produced  by  weak  stimuli  or 
in  certain  nervous  diseases,  they  develop  with  remarkable 
slowness. 

Pain  of  Non-cutaneous  Origin.  —  As  we  have  seen 
(page  12),  when,  owing  to  nerve  injury  or  disease,  the 
cutaneous  surface  has  become  analgesic,  pain  may  be  still 
produced  by  stimulating  deeper  structures.  For  aught  we 
know  to  the  contrary,  specific  end  organs  for  pain  may 
exist  in  the  subcutaneous  connective  muscular,  tendinous,  or 
other  tissue,  similar  to  those  which  are  doubtless  present  in 
the  skin. 

The  viscera,  whether  healthy  or  inflamed,  are  insensitive 
to  pain.  Cutting  or  burning  the  intestines  under  any 
conditions  is  a  painless  procedure,  so  long  as  the  sensitive 
parietal  peritoneum  is  shielded  from  the  stimulus.  The 
viscera  cause  pain  by  affecting  the  sensitive  parietal  peri- 
toneum, or  by  producing  what  has  been  called  "  referred " 
pain.  The  localisation  of  referred  pain  is  determined  by 
the  distribution  of  the  somatic  sensory  fibres,  which  belong 
to  the  same  spinal  segment  as  the  visceral  sensory  fibres 
supplying  the  visceral  area  in  question. 

The  Specific  Nature  of  Pain  Sensations. — The  discovery 
of  pain  spots  in  the  skin,  and  of  a  special  grouping  of  fibres 
conducting  painful  impulses  within  the  spinal  cord  (page 
14),  necessitates  a  reconsideration  of  the  older  view  that 
pain  may  result  from  the  excessive  stimulation  of  any 
sensory  nerve  whatever.  The  existence  of  painless  areas, 


1 6  EXPERIMENTAL  PSYCHOLOGY 

the  long  latency  and  the  high  threshold  of  pain  sensations 
neither  favour  nor  condemn  this  view.  Whether  it  must 
be  completely  abandoned,  or  whether  it  needs  only  to 
be  modified  in  the  direction  of  admitting  specific,  as  well 
as  non-specific,  pain  sensations,  the  evidence  at  present 
is  insufficient  to  decide.  A  like  uncertainty  attends  the 
conjecture  that  pain  is  merely  an  intenser  form  of  what 
is  loosely  termed  "general"  or  "common  sensibility"  or 
"  coensesthesia."  Experimental  psychology  has  left  this 
form  of  sensibility  practically  untouched. 

Temperature  Adaptation. — In  1846,  nearly  forty  years 
before  the  discovery  of  heat  spots  and  cold  spots,  Weber 
suggested  that  the  sensations  of  temperature  are  due  to  a 
rise  or  fall  in  the  temperature  of  the  skin  produced  by  the 
stimulus,  and  that,  consequently,  objects  which  are  of  the 
same  temperature  as  that  of  the  skin  appear  to  be  of  an 
indifferent  temperature.  This  view  is  for  many  reasons 
unsatisfactory.  The  after-effects  following  removal  of  a 
cold  stimulus  (exp.  13)  prove  that  the  sensation  of  cold 
may  persist  while  the  temperature  of  the  skin  is  rising. 
Moreover,  when  the  temperature  of  the  skin  remains  fairly 
constant,  as  in  prolonged  exposure  to  a  warm  fire  or  to  cold, 
the  sensation  of  warmth  or  cold  persists,  although  after  a 
time  the  temperature  of  the  skin  must  remain  practically 
unchanged. 

We  therefore  conclude  that  our  experience  of  tempera- 
ture is  dependent  not  upon  absolute  changes  in  the 
temperature  of  the  skin,  but  upon  the  relation  of  those 
changes  to  the  temperature  to  which  the  skin  is,  for  the 
time  being,  adapted.  Every  one  will  agree  that  at  a  given 
moment  the  temperature  of  different  parts  of  our  body  may 
feel  "indifferent,"  i.e.  neither  warm  nor  cold.  Yet  a 
thermometer,  applied,  say,  to  the  tongue  and  to  the  ear,  will 
register  about  37°  and  29°  C.  respectively.  It  is  also  a 
familiar  fact  that  the  temperature  of  a  room  which  feels 
warm  or  indifferent  in  winter,  will  appear  quite  cool  on  a 


CUTANEOUS  AND  VISCERAL  SENSATIONS     17 

hot  summer's  day.  These  are  extreme  cases,  but  they 
exemplify  the  general  rule  that  the  indifferent  temperature 
varies  according  to  the  temperature  to  which  the  body  or 
part  of  the  body  has  become  adapted. 

Kecognising  these  important  facts,  Hering  suggested 
that  sensations  of  temperature  are  the  result  of  raising  or 
lowering  the  body's  indifferent  temperature,  which  may 
itself  fluctuate  within  certain  fairly  wide  limits.  He 
termed  this  variable  indifferent  temperature  the  "  intrinsic  " 
or  "  adequate  "  temperature,  or  the  "  physiological  zero." 
It  is  the  temperature  to  which  the  body,  or  particular  area 
of  the  body,  is  for  the  time  being  adapted.  Further,  he 
suggested  that  sensations  of  heat  and  cold  may  be  attri- 
buted to  two  opposite  metabolic  processes,  of  "  dissimilation  " 
and  "assimilation,"  occurring  in  a  common  hypothetical 
temperature  apparatus.  During  the  state  of  adaptation, 
these  two  processes  are  both  excited  and  are  in  equilibrium, 
the  process  of  assimilation  (or  building  up)  being  equal  to 
the  process  of  dissimilation  (or  breaking  down).  Hering 
supposed  that  if  the  conditions  of  adaptation  be  now 
disturbed,  say  by  a  rise  of  temperature,  the  dissimilation 
process  at  first  preponderates,  and  hence  a  sensation  of 
heat  results.  Dissimilation,  however,  proceeds  faster 
than  regeneration  of  the  dissimilated  substances  can  occur 
within  the  temperature  apparatus.  Hence  the  amount 
of  dissimilation  becomes  less  and  less,  although  the  exciting 
temperature  change  persists.  Ultimately,  the  dissimilation 
process  is  so  far  reduced,  that  it  is  balanced  by  the  process 
of  assimilation  which  has  meanwhile  always  been  operative 
in  some  slight  degree. 

Thus  a  new  state  of  equilibrium  is  attained, — a 
state  of  -equilibrium  at  a  low  level  of  metabolism,  since 
nearly  all  the  store  of  material  available  for  dissimilation 
has  been  spent.  Hering  supposed  that  (within  certain 
limits)  equilibrium  may  be  produced  at  high  as  well  as  at 
low  levels  of  metabolism,  and  he  suggested  that  such 


1 8  EXPERIMENTAL  PSYCHOLOGY 

changes  in  the  position  of  equilibrium  correspond  to 
changes  in  the  physiological  zero,  the  temperature  of 
adaptation. 

Since  Hering's  theory  was  propounded,  separate  "  spots  " 
have  been  discovered  for  heat  and  for  cold,  and  it  has 
become  difficult  to  reconcile  the  peripheral  isolation  of  the 
end  organs  with  the  requirements  of  the  theory.  But  the 
quite  recent  discovery  of  an  additional  "  epicritic "  me- 
chanism, concerned  in  developing  sensations  of  warmth  and 
coolness  (page  12),  gives  a  possible  anatomical  basis  for  the 
occurrence  of  a  balance  between  assimilation  and  dissimila- 
tion within  a  single  peripheral  sensory  apparatus.  For  we 
have  evidence  that  while  heat  and  cold  spots  are  incapable 
of  adaptation,  adaptation  is  an  important  determinant  of  the 
temperature  sensations  derived  from  the  epicritic  system. 

Whether  we  accept  or  reject  Hering's  theory,  we  cannot 
neglect  the  facts  which  it  attempts  to  embrace.  We  must 
always  bear  in  mind  that  the  effect  of  a  thermal  stimulus 
upon  sensation  depends  not  merely  on  the  intensity,  dura- 
tion, and  extent  of  the  stimulus  (exp.  10),  but  also  on  the 
temperature  to  which  the  skin  is  at  the  moment  adapted 
(exp.  11). 

Touch  Adaptation. — It  is  interesting  to  note  that  similar 
conditions  of  adaptation  prevail  in  our  experiences  of  touch. 
The  tactual  sense  becomes  adapted  to  the  pressure  of  our 
clothes,  to  novel  pressures,  e.g.  when  eyeglasses  are  first 
worn,  and  to  the  removal  of  normal  pressures,  as  when  a 
tooth  is  extracted. 

Other  Cutaneous  Experiences. — Our  analysis  of  such 
experiences  as  roughness,  smoothness,  dryness,  wetness  is 
as  yet  too  imperfect  for  us  to  decide  as  to  their  nature.  In 
each  we  are  probably  dealing  with  the  effects  of  summation 
and  fusion  of  various  elementary  sensory  processes. 

Tickling  appears  to  depend  on  cutaneous  sensations 
arising  not  only  from  touch,  but  also  from  the  reflex  con- 
tractions of  the  unstriated  muscle  fibres  of  the  skin.  The 


CUTANEOUS  AND  VISCERAL  SENSATIONS     19 

peculiar  feeling  tone,  the  irradiating  character,  and  the 
tendency  to  produce  far-reaching  reflex  actions  are  remark- 
able, both  in  tickling  and  in  itching. 


BIBLIOGRAPHY. 

E.  H.  Weber,  "  Der  Tastsinn  und  das  Geraeingef iihl, "  in  Wagner's 
Handworterbuch  d.  PhysioL,  1846,  iii.  (2),  481  (reprinted  separately  by 
Engeliuann,  Leipzig).  E.  Hering,  "  Der  Tempera tursinn,"  in  Hermann's 
Handbuch  d.  PhysioL,  1880,  iii.  (2),  415.  H.  Donaldson,  "On  the 
Temperature  Sense,"  Mind,  1885,  x.  399.  M.  von  Frey,  "Beitr.  zur 
Sinnesphysiol.  d.  Haut,"2?er.  d.  sticks.  Gesell.  d.  Wiss.  (Math.-pliys.  Classe), 
1894,  1895,  1897.  A.  Goldscheider,  Gesammelte  Abhandlungen,  Leipzig, 
1898,  i.  C.  S.  Sherrington,  "  Cutaneous  Sensations,"  in  ScMfer's  Text-look 
of  PhysioL,  Edinburgh  and  London,  1900,  ii.  920.  T.  Tlmnberg,  "Physi- 
ologic d.  Druck,  Temperatur  und  Schmerzempfindungen,"  in  NageVs  Hand- 
buch d.  PhysioL  d.  Menschcn,  Braunschweig,  1905,  iii.  647.  H.  Head, 
W.  H.  R.  Rivers,  and  J.  Sherren,  "The  Afferent  Nervous  System  from  a 
New  Aspect,"  Brain,  1905,  xxviii.  99.  S.  Alrutz,  "Die  Kitzel-  u. 
Juckempfindungen,  Skand.  Arch.  f.  PhysioL,  1908,  xx.  371. 


CHAPTEE    III 
ON    AUDITORY   SENSATIONS  1 

The  Physical  Basis  of  Pitch  and  Loudness. — In  general, 
our  auditory  sensations  are  due  to  the  occurrence  of  sound 
waves  in  the  external  air.  These  waves  vary  in  length,  in 
amplitude,  and  in  form.  Other  things  being  equal,  the 
shorter  the  wave  length  (i.e.  the  greater  the  vibration 
frequency)  the  higher  in  pitch  will  be  the  auditory  sensa- 
tion ;  and  the  greater  the  amplitude  (i.e.  the  more  distant 
the  crest  or  trough  of  a  wave  from  the  position  of  equilibrium) 
the  louder  or  more  intense  will  be  the  auditory  sensation. 
But  these  relations  are  only  broadly  true.  For,  as  we  shall 
see,  the  pitch  and  the  loudness  of  our  sensations  do  not 
always  correspond  to  the  wave  length  and  the  amplitude  of 
the  objective  stimuli. 

[The  Conduction  of  Sounds  to  the  Inner  Ear. — Sounds 
which  are  of  moderate  pitch  and  loudness  are  led  to  the 
inner  ear  by  the  membranes  and  ossicles  of  the  middle  ear. 
It  has  been  experimentally  shown  that  the  ossicles  vibrate 
with  a  frequency  dependent  on  the  vibration  frequency  of 
the  sound  stimulus,  and  that  they  must  consequently  be 
regarded,  not  as  a  fixed  conducting  rod,  but  as  a  jointed, 
freely  vibrating  chain. 

If,  however,  the  sounds  be  loud  enough  or  be  of 
sufficiently  high  pitch,  they  are  audible  to  persons  who 
through  injury  or  disease  have  altogether  lost  this  mem- 

1  The  student  is  recommended  to  omit  at  his  first  reading  those  portions 
which  are  enclosed  in  square  brackets  [  ]. 

20 


AUDITORY  SENSATIONS  21 

branous  and  bony  apparatus  of  the  middle  ear.  It  has  been 
demonstrated  that  high  or  very  loud  sounds  are  directly 
communicable  from  the  air  to  the  inner  ear  by  way  of  the 
bony  walls  of  the  skull.  Under  normal  as  well  as  under 
abnormal  conditions  of  health,  there  can  be  little  doubt 
about  the  occurrence  of  such  direct  bone  conduction  in  the 
case  of  high  or  very  loud  sounds.] 

[  The  Conduction  of  Sounds  from  Ear  to  Ear. — Direct  bone 
conduction  also  occurs  when  a  sounding  instrument,  e.g.  a 
tuning-fork,  is  brought  in  contact  with  the  head  or  teeth 
(exp.  15).  It  is  likewise  a  factor  of  considerable  psycho- 
logical importance  when  a  sound  is  led  to  one  ear  only 
(exp.  16);  for  unless  the  sound  be  low  in  pitch  and 
intensity,  it  travels  to  the  opposite  ear,  partly  perhaps  by 
way  of  the  two  Eustachian  tubes  across  the  pharynx,  but 
chiefly  over  the  bony  vault  and  across  the  base  of  the  skull. 
For  this  reason  we  are  practically  unable  to  excite  the 
auditory  end  organs  of  one  side  of  the  body  without 
simultaneously  exciting  (of  course,  in  less  degree)  the 
corresponding  organs  of  the  opposite  side,  —  an  experi- 
mental difficulty  which  it  is  most  important  to  bear  in 
mind.] 

The  Physical  Basis  of  Timbre. — We  have  seen  that  the 
pitch  and  the  loudness  of  sensations  of  sound  are  closely 
connected  with  wave  length  and  amplitude,  but  we  have 
yet  to  examine  a  feature  in  which  sound  waves  further 
differ  from  one  another,  namely,  variety  of  form.  The  vibra- 
tions of  a  sound  wave,  if  wholly  devoid  of  regularity, 
are  termed  "non-periodic."  They  are  termed  "periodic" 
when  during  equal  periods  of  time,  however  long,  the 
same  movements  are  repeated,  however  complex.  Further, 
periodic  vibrations  are  classified  as  "  pendular  "  and  "  non- 
pendular." 

Pendular  vibrations  produce  a  particular  form  of  sound 
wave,  the  "  sine  wave,"  which  is  important  because  theoreti- 
cally it  gives  rise  to  the  purest  sensation  of  a  single 


22  EXPERIMENTAL  PSYCHOLOGY 

tone.1  All  other  periodic  vibrations  are  non-pendular.  It  is 
commonly  stated  that  non-peridular  periodic  vibrations  pro- 
duce composite  or  complex  tone  sensations,  and  that  non- 
periodic  vibrations  produce  sensations  of  noise.  We  shall 
presently  see  that  these  statements  are  only  broadly  true ; 
but  before  we  can  advantageously  study  the  psychological 
effects  of  pendular,  non-pendular,  and  non-periodic  vibrations, 
it  is  important  to  grasp  the  significance  of  a  mathematical 
theorem  and  of  a  physical  principle  of  acoustics,  which  are 
closely  connected  with  the  physiology  and  psychology  of 
hearing.  These  are  "  Fourier's  theorem  "  and  the  "  principle 
of  resonance." 

Fourier's  Theorem. — This  theorem  states  that  a  non- 
pendular  periodic  wave  may  always  be  resolved  into  a  series 
of  pendular  waves.  The  longest  of  these  pendular  waves 
has  the  same  frequency  as  the  non-pendular  wave,  and  the 
other  pendular  waves  of  the  series  have  2,  3,  4  ...  times 
that  frequency.  Any  one  or  more  members  of  such  a  series 
may  be  missing.  The  number  of  members  is  commonly 
infinite,  but  the  amplitude  of  the  higher  members  is  usually 
so  small  that  the  first  few  component  waves  suffice  to  give 
an  approximation  to  the  original  non-pendular  wave. 

Resonance. — The  principle  of  resonance  or  of  sympathetic 
vibration  concerns  objects  that  have  a  natural  rate  of 
vibration  which  they  execute  more  readily  than  any  other 
rate.  A  confined  chamber  of  air,  or  a  stretched  string, 
forms  an  admirable  resonator.  Let  us,  for  purposes  of 
illustration,  choose  the  latter  object,  and  let  us  suppose  that 
its  tension  is  such  that,  when  struck  or  bowed,  it  executes 
two  hundred  vibrations  per  second,  emitting  a  tone  of  this 
vibration  frequency.  Let  us  suppose  that  the  string  is  at 

1  This  form  of  sound  wave  is  called  the  "  sine  wave"  because  at  any 
instant  the  displacement  of  any  particle  from  its  position  of  equilibrium  is 
proportional  to  the  sine  of  an  angle,  which  in  turn  is  proportional  to  the 
distance  of  that  particle  from  the  rear  of  the  wave.  Within  the  length  of  a 
wave  this  angle  increases  from  o  to  2ir, 


AUDITORY  SENSATIONS  23 

rest,  and  that  sound  waves,  having  precisely  this  vibration 
frequency,  now  travel  to  it  through  the  air,  generated  from 
some  extraneous  source.  The  resting  string  will  at  once 
vibrate  sympathetically.  It  is  thrown  into  its  natural  rate 
of  vibration,  resonating  to  the  vibrations  of  the  air. 

The  principle  of  resonance  can  be  best  understood  by 
comparing  the  resonant  object  to  a  freely  swinging  pendulum. 
Imagine  a  pendulum  of  such  a  length  that  it  executes  a 
complete  oscillation  (to  and  fro)  in  one  second.  Let  it  start 
from  at  rest,  and  receive  a  series  of  infinitesimally  minute 
taps  which  are  regularly  given  at  intervals  of  a  second. 
Then  every  tap,  if  given  at  the  very  start  of  a  pendular 
excursion,  will  serve  to  increase,  ever  so  slightly,  the  extent 
of  the  following  excursion ;  the  ultimate  result  being  that 
the  pendulum  will  swing  with  a  very  wide  excursion.  It 
is  just  the  same  in  the  case  of  our  example  of  the  resonating 
string,  the  natural  vibration  period  of  which  is  two  hundred 
times  more  rapid  than  that  of  the  pendulum.  The  regular 
condensations  (and  rarefactions)  of  the  advancing  sound 
waves  correspond  to  the  taps  received  by  the  pendulum.  A 
succession  of  such  minute  thrusts  (and  pulls),  administered 
with  appropriate  frequency,  finally  produces  relatively 
powerful  vibrations  in  the  resonating  string. 

If  the  rate  of  taps  given  to  the  pendulum  do  not  accu- 
rately accord  with  its  natural  rate  of  swing,  their  effect,  of 
course,  is  not  so  favourable.  So,  too,  the  string  resonates 
more  and  more  feebly,  the  less  exact  be  the  correspondence 
between  the  pitch  of  the  reinforcing  tone  and  the  natural 
vibration  rate  of  the  string ;  until  ultimately,  when  the  dis- 
crepancy is  too  great,  there  will  be  no  resonance  effect  at 
all.  When  a  string  can  be  forced  to  resonate  at  all  by  such 
non-corresponding  vibrations,  it  vibrates  in  the  period  of 
the  latter,  but  it  immediately  returns  to  its  natural  period 
of  vibration  when  those  vibrations  cease  to  act  on  it. 

We  have  already  observed  (page  21)  that  a  pure  tone 
sensation  is  theoretically  produced  by  the  pendular  vibrations 


24  EXPERIMENTAL  PSYCHOLOGY 

of  a  sine  wave.  Let  us  now  suppose  that  several  sine 
waves  of  different  wave  lengths  are  brought  simultaneously 
before  a  single  resonator.  In  other  words,  let  us  suppose 
that  several  pure  tones  of  different  pitch  are  simultaneously 
sounded  before  a  single  resonator.  The  resonator  will 
immediately  pick  out  that  tone  to  which  it  is  attuned, 
resonating  powerfully  thereto.  A  moment's  consideration 
will  show  that  the  sound  vibrations  acting  on  the  resonator, 
although  periodic,  are  no  longer  pendular;  they  are  the 
resultant  of  the  simultaneous  movements  imparted  to 
the  air  by  pendular  waves  of  different  length.  But  the 
resonator  has  the  power  of  analysing  these  complex  periodic 
non-pendular  vibrations ;  it  responds  to  that  particular 
pendular  component  to  which  it  is  attuned.  Only  a  suffi- 
ciently large  series  of  appropriate  resonators  is  needed  in 
order  completely  to  analyse  a  number  of  simultaneously 
sounding  tones,  however  great  (exps.  17,  18). 

On  the  mathematical  side,  this  physical  resolution  of 
mixed  tones  finds  its  counterpart  in  Fourier's  theorem, 
to  which  we  need  not  refer  again  here.  On  the  psycho- 
logical side,  it  is  a  matter  of  common  knowledge  that  the 
practised  musician  can  readily  analyse  a  group  of  simul- 
taneously sounding  tones  into  its  components,  if  they  be 
not  too  numerous  or  of  too  nearly  identical  pitch.  It  was 
this  similarity  between  physical  and  psychical  behaviour 
that  led  Helmholtz  to  suppose  that  the  cochlea  contains  a 
series  of  resonating  fibres,  differently  attuned,  each  selecting 
its  appropriate  pendular  constituent  of  the  non-pendular 
periodic  vibrations  reaching  the  inner  ear.  We  shall  later 
(page  51)  discuss  this  theory  of  hearing  in  detail. 

Most  resonators,  besides  being  capable  of  vibrating  as  a 
whole,  may  vibrate  in  halves,  in  thirds,  in  quarters,  etc., 
each  section  or  inter  node  executing  twice,  three,  four,  etc., 
times  as  many  vibrations  per  second  as  the  entire  resonator. 
Thus,  a  resonating  string,  or  a  hollow  tube,  may  be  thrown 
into  sympathetic  vibration  by  sound  waves,  the  period  of 


AUDITORY  SENSATIONS  25 

which  is  any  simple  multiple  of  the  vibration  frequency  to 
which  the  entire  string  or  chamber  is  attuned. 

[The  Middle  Ear. — We  are  now  in  a  position  to  study 
the  function  of  the  parts  within  the  middle  ear.  The 
tympanic  membrane  is  drawn  inwards  at  its  central  region 
by  the  handle  of  the  malleus,  to  which  it  is  attached  by  its 
layer  of  radially  disposed  fibres.  The  malleus  is  in  turn 
pulled  inwards  by  the  action  of  the  tensor  tympani  muscle, 
and  the  radial  fibres  of  the  tympanic  membrane  are  bent 
convexly  outwards,  chiefly  owing  to  the  constricting  action 
of  the  layer  of  circular  fibres. 

Experiments  have  shown  that  such  a  curved  membrane 
has  the  advantage  of  possessing  only  feeble  powers  of  reson- 
ance. Were  the  tympanic  membrane  flatter,  denser,  and 
freer,  it  would  resonate  more  powerfully  to  one  particular 
tone  than  to  others, — a  feature  obviously  harmful  to  good 
hearing.  Its  peculiar  form  and  connections  allow  it  to 
respond  fairly  equally  to  the  wide  range  of  tones  to  which 
the  ear  is  sensible  (exp.  19). 

The  tensor  tympani  muscle  contracts  when  the  sounds 
are  very  high  or  loud.  It  has  been  supposed,  on  insufficient 
grounds,  that  by  increasing  the  curvature  and  tension  of 
the  tympanic  membrane,  this  muscle  protects  the  membrane 
from  excessive  vibration,  and  thus  from  risk  of  damage.  It 
has  also  been  supposed  that  the  tensor  tympani  muscle  con- 
tracts with  different  force  according  to  the  pitch  of  the  tone 
stimulus,  tuning  the  membrane  so  that  it  responds  with 
maximal  sensitiveness  to  tones  of  different  pitch.  But  the 
experiments  that  have  been  advanced  in  favour  of  this 
accommodatory  function  of  the  muscle  are  not  convincing. 
They  were  performed  on  dead  animals,  the  tension  of  the 
membrane  being  varied  by  applying  different  degrees  of 
artificial  traction  to  the  tendon  of  the  muscle. 

The  position  of  the  stapes  is  controlled  by  the  stapedius 
muscle,  which  contracts  during  attentive  listening.  Its 
action  is  to  pull  the  ossicles  towards  the  tympanic  mem- 


26  EXPERIMENTAL  PSYCHOLOGY 

brane,  lessening  the  curvature  of  the  latter.  Consequently 
the  pressure  within  the  inner  and  middle  ear  falls,  the 
tympanic  membrane  vibrates  more  freely,  and  the  keenness 
of  hearing  is  increased.  On  the  other  hand,  the  pressure 
within  the  middle  ear  is  raised  during  yawning  (owing  to 
the  accompanying  contraction  of  the  tensor  tympani),  or 
when  air  is  forced  through  the  Eustachian  tube  into  the 
tympanic  cavity :  in  either  case  a  marked  damping  of  loud 
and  deep  tones  results.] 

[The  Relations  of  Noise  to  Tone. — In  order  that  pendular 
vibrations  of  the  air  may  produce  a  sensation  of  tone,  their 
number  that  reaches  the  ear  must  not  fall  below  a  certain 
limit.  Below  that  limit  the  auditory  sensation  produced 
by  the  vibrations  is  one  not  of  tone,  but  of  noise,  in  which  it 
is  impossible  to  recognise  a  precise  pitch.  According  to  the 
methods  employed,  the  various  attempts  which  have  been 
made  to  determine  the  minimal  number  of  vibrations 
necessary  for  a  tone  sensation  have  differed  considerably  in 
result.  A  serious  obstacle  to  the  success  of  such  experi- 
ments lies  in  the  generation  of  secondary  sound  waves 
arising  from  reflexion  and  other  causes.  The  investigator 
can  never  be  satisfied  that  the  exact  number  of  vibrations 
which  he  has  so  carefully  produced  will  not  be  augmented 
by  a  train  of  others  before  the  inner  ear  is  reached.  Several 
recent  observers,  however,  agree  that  throughout  a  consider- 
able range  of  tones  only  two  pendular  vibrations  need  be 
generated  in  order  to  produce  a  tone  sensation.  But  more 
than  two  vibrations  are  required  in  order  that  the  pitch  may 
be  with  certainty  recognised. 

Hence  it  is  incorrect  to  say  that,  whereas  noise  results 
from  non-periodic  vibrations,  sine  waves,  i.e.  periodic  vibra- 
tions, always  produce  sensations  of  tone.  "We  have  just 
seen  that  if  the  sine  waves  be  too  few  in  number,  a  sensa- 
tion of  noise,  not  of  tone,  results.  Further,  when  various 
sine  waves  are  simultaneously  produced,  which  are  nearly, 
but  not  quite,  of  identical  wave  length,  a  sensation  of  noise 


AUDITORY  SENSATIONS  27 

is  produced.  This  effect  is  easily  obtainable  by  pressing 
two  long  boards  on  the  keys  of  the  pianoforte,  so  as  to 
strike  a  series  of  neighbouring  black  and  white  notes  simul- 
taneously. But  the  effect  is  still  noisier,  if  the  tones 
sounded  differ  less  in  pitch  than  those  of  this  instrument 
(exp.  20). 

Noises  have  been  divided  into  two  classes — momentary 
and  continuous.  A  momentary  noise,  e.g.  the  noise  of  an  ex- 
plosion or  of  an  electric  spark,  is  sometimes  stated  to  be  the 
effect  of  a  single  sound  wave.  Continuous  noises,  e.g.  the  rust- 
ling of  leaves  or  the  roar  of  waves,  are  sometimes  described 
as  the  sum  of  small  momentary  noises.  But  the  objections, 
which  we  have  just  urged,  against  the  action  of  single 
sound  waves  are  also  valid  here. 

Having  reviewed  the  physical,  we  may  pass  on  to  the 
psychological  relations  between  tone  and  noise.  A  discus- 
sion of  their  physiological  relations  will  be  deferred  until 
we  come  to  consider  the  general  theories  of  hearing 
(page  55).  It  is  often  said  that  a  noise  is  as  different  from 
a  tone  sensation  as  white  is  different  from  a  colour  sensa- 
tion. Certainly,  in  their  purest  form,  noise  and  tone  are 
fundamentally  different  experiences.  The  one  is  unpleasant, 
rough,  irregular,  and  difficult  to  analyse ;  the  other  is 
pleasant,  smooth,  regular,  and  relatively  simple. 

It  is  clear  that  every  gradation  may  occur  between  a 
sensation  of  noise  and  one  of  tone,  e.g.  between  the  noise 
resulting  from  rubbing  a  table  and  the  tone  resulting  from 
rubbing  a  finger-glass.  One  reason  for  the  possibility  of 
this  gradation  lies  in  the  fact  that  there  are  few  noise 
stimuli  that  do  not  contain  tone  stimuli  also.  Tones  of 
definite  pitch  may  be  detected  in  the  roar  of  a  waterfall  or 
amid  the  hum  of  traffic  in  the  heart  of  a  busy  city. 
Eesonators  will  help  in  the  identification  of  such  tonal 
components  of  noises.  But  even  a  quite  toneless  noise  is 
not  devoid  of  pitch,  although  the  latter  can  be  estimated 
only  vaguely.  For,  after  a  little  practice,  we  can  definitely 


28  EXPERIMENTAL  PSYCHOLOGY 

say  that  one  noise  is  higher  or  lower  than  another,  although 
we  are  unable  to  determine  its  precise  pitch  (exp.  20).] 

Timbre. — If  nearly  all  noises  contain  tones,  it  is  likewise 
true  that  the  production  of  tones  involves  noise.  It  is 
impossible  to  name  an  instrument  which  can  emit  a  noise- 
less tone.  In  the  horn  noise  arises  from  blowing,  in  the 
piano  from  striking,  in  the  harp  from  plucking,  and  in  the 
violin  from  bowing  the  instrument. 

Now  it  is  obvious  that  the  sounds  of  these  several 
instruments  differ  from  one  another,  apart  from  their  specific 
accompanying  noises.  A  tone  of  a  given  pitch  on  the 
bassoon  is  not  to  be  mistaken  for  a  tone  of  the  same  pitch 
on  the  piano  or  on  the  violoncello.  The  difference  does 
not  lie  in  noise,  pitch,  or  loudness,  but  in  a  quality  which 
we  call  "  timbre"  (exp.  21). 

Tone  Nomenclature. — Before  we  examine  the  basis  of 
this  experience  of  timbre,  it  is  convenient  to  describe  the 
nomenclature  usually  applied  to  the  range  of  tones.  The 
middle  c  on  the  pianoforte  is  written  c' ';  its  vibration 
frequency  is  256  per  second.1  The  next  c,  with  a  vibration 
frequency  of  512,  is  written  c",  the  c  above  which  (1024 
vibrations)  is  written  c'",  and  so  on.  The  c  below  c'  is  c°, 
the  c  below  this  is  C0,  below  which  are  C^  and  Cz.  The 
"  major  scale  "  from  c'  to  c"  is  therefore  written — 

c         d'        ef         f         gf         a'          lf        c" 
256     288     320     341-3     384    426'6     480     512 

The  octave  in  which  this  scale  is  written  is  called 
the  once-accented  octave.  Above  it  come  the  twice,  three 
times,  four  times,  etc.,  accented  octaves.  Below  it  the 
octaves  are  successively  named  the  small,  the  great,  the 
contra-  and  the  subcontra-octaves. 

[The  figures  below  the  above  notes  give  the  vibration 
frequency.  The  relation  of  c'  to  c"  (256  :  512  =  1  :  2)  is  that 

1  This  is  the  pitch  of  c'  adopted  for  scientific  purposes.  As  a  matter  of 
fact,  the  middle  c  of  the  English  pianoforte  has  a  vibration  frequency  of 
about  270, 


AUDITORY  SENSATIONS  29 

of  a  tone  to  its  "  octave  " ;  c'  :  g'  (2  :  3)  forms  the  interval 
of  a  "  fifth  " ;  c'  :  f  (3  :  4)  forms  a  "fourth " ;  c  :  e'  (4  :  5) 
forms  a  "  major  third "  ;  c'  :  a'  (3  :  5)  a  "  major  sixth." 
The  above  together  with  the  "  prime "  (cf  :  c =1  :  1),  the 
"  minor  third  "  (c'  :  &  =  5  :  6)  and  the  "  minor  sixth  "  (c1  :  a* 
=  5  :  8),  are  the  "consonant"  or  pleasing  intervals  within 
the  octave.  The  "dissonant"  or  disagreeable  intervals 
include  the  "major  second,"  c'  :  d'  (8  :  9),  the  "minor 
second,"  c'  :  d*  (15  :  16),  the  "major  seventh,"  c  :  V 
(8  :  15), and  the  "minor  seventh 'V  :  6/b(5  :  9).  Mention 
may  also  be  made  of  an  "  augmented  fourth "  c  :  f'* 
(32  :  45),  called  the  "  tritone,"  which  is  nearly  identical 
with  the  ratio  5  :  7.  The  latter  interval  and  the  interval 
4  :  7,  which  do  not  occur  in  our  musical  scales,  are  inter- 
mediate between  the  consonances  and  dissonances.  Intervals 
wider  than  the  octave,  e.g.  the  "  twelfth  "  (1  :  3)  c'  :  g",  or 
the  "  major  ninth  "  (4  :  9)  c'  :  d",  have  the  same  relations 
as  corresponding  intervals  (c'  :  /,  c'  :  d')  within  the  octave. 

The  first  note  of  the  scale  is  called  the  "  tonic."  In  the 
major  scale,  described  above,  all  the  intervals  reckoned  from 
the  tonic  are  major.  Minor  intervals  appear  only  in  the 
minor  scales.  Let  us  for  the  moment  restrict  our  attention 
to  the  major  scale,  and  let  us  proceed  to  construct  another 
major  scale,  making  d  the  tonic  in  place  of  c,  and  calculating 
the  vibration  numbers  of  the  tones  that  lie  a  major  second, 
a  major  third,  a  fourth,  a  fifth,  a  major  sixth,  and  a  major 
seventh  above  the  tonic.  Then  we  shall  find  that  only  two 
of  these  six  tones  are  already  represented  in  the  scale  of  c. 
When  further  we  similarly  calculate  the  tones  of  the  major 
scales  of  e,  /,  g,  a,  and  b,  we  find  that  altogether  eleven 
additional  tones  are  required  within  the  octave.  So,  too,  in 
order  to  play  the  minor  intervals  it  will  be  found  that  eight 
additional  tones  are  required. 

Now,  in  order  to  reduce  the  enormous  number  of  notes 
which  would  thus  be  needful  for  us  to  play  major  and 
minor  scales  from  all  possible  tonics,  a  system  of  equal 


30  EXPERIMENTAL  PSYCHOLOGY 

temperament  has  been  employed  in  tuning  the  pianoforte 
and  similar  instruments.  The  tones  between  each  octave 
are  divided  into  twelve  equidistant  semitones,  the  result 
of  which  is  that  the  only  exactly  intoned  interval  is  the 
octave.  All  the  other  intervals  in  such  tempered  instru- 
ments are  more  or  less,  but  so  slightly,  out  of  tune,  that 
we  can  start  a  scale  on  any  tonic  we  please.  In  other 
words,  we  can  play  a  melody  in  any  key  we  like.] 

The  Relation  of  Overtones  to  Timbre. — Hitherto  we  have 
spoken  of  pendular  vibrations  of  sound  waves  and  of  the 
corresponding  pure  tone  sensations  as  if  they  had  real 
existence.  We  have  now  to  qualify  this  notion  very 
materially.  There  is  no  known  source  of  sound  that  pro- 
duces a  pure  tone  sensation.  If  an  instrument  be  sounded 
so  as  to  produce  a  tone,  say  of  500  vibrations  per  second, 
we  cannot  avoid  the  simultaneous  production  of  other  tones 
which,  in  many  cases,  are  simple  multiples  of  that  tone. 
We  speak  of  the  tone  500  as  the  "primary"  or  "funda- 
mental "  tone.  The  tones  which  accompany  it,  having 
vibration  frequencies  of  1000,  1500,  2000,  etc.,  are  called 
"harmonics"  or  harmonic  "overtones."  The  number  and 
loudness  of  these  various  overtones  vary  in  different  instru- 
ments, and  determine  the  "  colour  "  or  timbre  of  the  tones 
produced.  A  little  care  will  enable  even  the  unmusical 
observer  to  detect  various  overtones  in  a  note  of  the  piano 
or  violin.  By  means  of  resonators  the  overtones  may  be 
accurately  indentified  (exp.  22).  It  is  found  that  the  dull 
sounds  of  the  flute  are  poor  in  overtones,  that  the  piercing 
sounds  of  brass  instruments  are  especially  rich  in  the  higher 
overtones,  and  that  the  nasal  quality  of  the  clarionet  is  due 
to  the  feebleness  of  the  odd  series  of  overtones  (first,  third, 
etc.),  and  to  the  loudness  of  the  higher  members  of  the  even 
series.  Similar  analyses  have  been  applied  to  the  vowel 
sounds  of  our  voice.  One  vowel  differs  from  another  of 
the  same  pitch  owing  to  the  number  and  loudness  of  the 
overtones  generated  (exp.  23).  Indeed,  by  combining  a 


AUDITORY  SENSATIONS  31 

number  of  appropriate  tones  of  suitable  loudness,  we  may 
synthetically  produce  different  vowels  with  very  fair 
success. 

The  tuning  -  fork  is  chosen  for  auditory  experiments 
because  of  the  great  purity  of  its  tone.  But  even  here 
the  first  overtone  may  be  sometimes  detected,  while  other 
overtones  which  are  not  simple  multiples  but  "  inharmonic," 
are  often  present  as  in  many  other  instruments. 

[A  tone  which  is  poor  in  overtones  not  only  has  a  duller 
timbre,  but  actually  appears  to  be  lower  in  pitch  than  a 
tone  rich  in  overtones.  This  fact  has  been  used  to  explain 
the  rise  of  pitch  which  is  observable  as  the  tone  of  a  tuning- 
fork  dies  away,  or  as  a  fork  is  slowly  withdrawn  from  a 
position  close  to  the  ear.  Distance  is  believed  to  affect  the 
apparent  strength  of  a  primary  tone  more  than  that  of  its 
overtones. 

But  if  the  timbre  of  the  tones  produced  by  various 
orchestral  instruments  depends  on  the  nature  and  loudness 
of  the  overtones,  we  may  well  wonder  how  we  can  determine 
that  a  given  tone  or  a  phrase  in  a  piece  of  orchestral  music 
is  being  played  by  a  particular  instrument.  The  various 
tones  and  overtones  reach  the  ear  in  complete  disorder. 
How  then,  we  may  ask,  can  the  overtones  be  sorted  out 
and  allotted  to  the  fundamental  tone  to  which  they  belong 
so  that  that  tone  regains  its  original  colour  ?  In  point  of 
fact,  even  the  most  experienced  musican  is,  under  certain 
conditions,  liable  to  erroneous  judgment.  But  our  ability, 
remarkable  as  it  is,  is  chiefly  due  to  past  experience,  to 
differences  in  the  position  of  instruments  in  the  orchestra, 
to  movements  of  our  head  whereby  we  are  able  to  vary 
concurrently  the  loudness  of  the  tones  and  of  the  overtones 
of  any  one  instrument,  and  to  the  different  course  of  the 
phrases  of  tones  played  by  different  instruments  of  the 
orchestra.  The  last-named  factor  is  doubtless  of  extreme 
importance.  For  every  variation  in  time,  loudness,  or  pitch 
which  the  fundamental  tones  of  an  instrument  undergo 


32  EXPERIMENTAL  PSYCHOLOGY 

must  be  similarly  shared  by  their  overtones.  This  provides 
a  means  of  allotting  the  overtones  to  their  proper  funda- 
mental tones.] 

[The  Threshold  of  Intensity. — In  order  that  a  tone  or  a 
noise  may  be  heard,  the  intensity  of  the  stimulus  must  not 
fall  below  a  limiting  or  "  liminal "  value.  The  value  of  this 
"  limen  "  or  "  threshold  "  varies  according  to  the  nature  of 
the  sound,  the  simultaneous  presence  of  other  sounds,  the 
acuity  of  the  subject's  hearing,  and  the  method  of  estimating 
the  threshold.  We  shall  refer  again  to  these  conditions 
when  we  come  to  treat  of  sensory  acuity  and  attention 
(chapters  xviii.  and  xxiv.). 

If,  on  the  other  hand,  the  loudness  of  the  purest  obtain- 
able tone  be  too  great, — that  is,  if  the  degree  of  displace- 
ment of  the  vibrating  particles  from  equilibrium  be  too 
great  relatively  to  the  magnitude  of  the  force  that  displaces 
them, — overtones  are  produced  and  the  tone  no  longer 
preserves  its  original  purity. 

There  is  a  general  tendency  for  the  pitch  of  loud  sounds 
to  appear  too  high,  and  conversely  for  the  pitch  of  faint 
sounds  to  appear  too  low.  This  illusion  is  partly  due  to  the 
change  of  timbre  (page  28)  arising  from  differences  in  the 
ratio  of  the  fundamental  tone  to  the  overtones.  It  is, 
however,  doubtless  also  connected  with  the  greater  psychical 
intensity  of  high  tones  as  compared  with  that  of  low 
tones  (page  35). 

Before  a  sound  stimulus  produces  a  sensation,  it  has  to 
overcome  what  we  may  describe  as  the  inertia  of  the 
auditory  apparatus.  If  we  place  the  ends  of  a  rubber  tube 
one  in  either  ear,  and  if  we  gently  rest  a  very  weakly 
vibrating  tuning-fork,  preferably  of  low  pitch,  on  the  mid- 
point of  the  tube,  the  tone  will  at  first  be  inaudible,  will 
later  be  heard,  and  will  then  gradually  increase  in  loudness, 
the  sensation  reaching  its  maximum  after  one  or  two 
seconds.  The  time  thus  spent  in  evoking  an  auditory 
sensation  is  in  part  due  to  changes  produced  in  the  inner 


AUDITORY  SENSATIONS  33 

ear,  in  the  auditory  nerves,  and  in  the  cerebral  centres  with 
which  the  latter  directly  or  indirectly  communicate.  But 
it  is  chiefly  due  to  the  inertia  of  the  apparatus  of  the  middle 
ear,  which  has  to  be  overcome  in  order  that  the  resting 
ossicles  may  execute  their  proper  vibrations.  Persons 
whose  ossicles  vibrate  with  difficulty  owing  to  disease 
(whose  auditory  acuity  is  therefore  subnormal)  may  not  be 
able  to  hear  a  loudly  sounding  tuning-fork  until  it  has  been 
held  for  several  seconds  before  the  ear.  Such  subjects  can 
often  hear  distinctly  better  when  they  travel,  or  when 
they  are  in  a  room  where  noisy  conversation  is  being 
carried  on.  Apparently  the  rattle  of  the  carriage  or  the 
general  buzz  of  voices  stirs  their  ossicles  into  movement, 
so  that  the  latter  become  more  sensitive  to  auditory 
stimuli.] 

[Auditory  After-sensations. — Not  only  is  there  a  latent 
period  between  the  application  of  the  stimulus  and  the 
development  of  the  auditory  sensation,  there  is  also  a 
period  during  which  the  sensation  persists  after  the  with- 
drawal of  the  stimulus.  This,  again,  is  in  part  due  to  the 
persistence  of  peripheral  and  central  nervous  processes  ;  but 
it  also  arises  from  the  after-vibrations  of  the  ossicles  of  the 
middle  ear.  Unfortunately  the  attempts  hitherto  made  to 
determine  the  duration  of  these  "  after- sensations  "  are  of  too 
unsatisfactory  a  character  to  be  given  in  full  detail  here. 
The  usual  procedure  in  such  experiments  has  been  to  inter- 
rupt a  given  tone  stimulus,  or  to  alternate  two  different 
tone  stimuli,  so  rapidly  that  a  continuous  (or  an  interrupted) 
experience  is  just  produced.  The  varying  results  are  partly 
attributable  to  different  experimental  methods.  But  the 
chief  and  most  obvious  objection  to  such  experiments  lies  in 
the  fact  that  the  determination  of  any  given  after-sensation 
is  complicated  by  the  time  occupied  in  the  development 
of  the  following  sensation.  The  course  of  each  auditory 
sensation  must  be  regarded  as  a  curve,  slowly  reaching  its 
maximum  after  the  stimulus  is  first  presented,  and  declining 
3 


34  EXPERIMENTAL  PSYCHOLOGY 

gradually  after  the  stimulus  has  been  withdrawn.  When 
two  such  curves  rapidly  follow  so  as  partially  to  overlap 
one  another,  it  is  clearly  hopeless  to  endeavour  to  determine 
the  duration  of  one  component  of  one  curve,  if  the  duration 
of  other  components  of  the  other  be  quite  unknown. 

These  after-sensations  of  hearing  must  be  distinguished 
from  certain  continuous  or  intermittent  "  revived  sensations  " 
which  are  far  rarer  and  are  sometimes  referable  to  pathological 
disorder.  They  are  said  to  occur  more  frequently  among 
subjects  whose  hearing  is  subacute.  Their  onset  usually 
follows  some  fifteen  or  more  seconds  after  the  stimulus  has 
been  withdrawn,  but  this  is  not  always  the  case.  After 
prolonged  experiments  during  the  day  with  the  highest 
audible  tones,  they  may  recur  at  night  and  their  vividness 
may  be  so  intense  that  they  may  be  confused  with  sensations 
of  objective  origin.  In  most  instances  of  revived  sensations 
it  is  highly  improbable  that  they  are  due  to  after- vibrations 
of  the  ossicles  of  the  middle  ear.  In  some  cases  the  absence 
of  beats  (page  37)  or  difference  tones  (page  42),  when 
revived  sensations  of  different  pitch  are  simultaneously 
present,  points  to  a  neural  or  central  origin.  They  are 
probably  akin  to  visual  hallucinations,  and  are  perhaps  the 
result  of  abnormal  nervous  excitability  (exp.  24). 

Even  when  all  the  effects  of  a  sound  stimulus  have 
passed  away,  the  end  organs  of  the  ear  do  not  enjoy  com- 
plete rest.  They  are  perpetually  being  played  upon  by 
feeble  stimuli  of  internal  origin,  due,  for  example,  to  the 
circulation  of  the  blood  or  to  muscular  contractions  within 
the  ear ;  although  it  is  to  this  condition  that  we  give  the 
name  of  silence.  In  "  singing  in  the  ear  "  such  intra-aural 
disturbances  reach  a  distressing  intensity.] 

Tone  Character. — Tone  sensations  of  different  pitch  vary 
in  a  quality  which  we  may  call  "tone  character"  (exp.  25). 
A  very  high  tone  sensation  appears  thin,  pointed,  and 
piercing,  a  very  low  tone  sensation  appears  coarse,  volu- 
minous, and  massive.  These  special  characters  of  tone 


AUDITORY  SENSATIONS  35 

sensations  are  in  parb  dependent  on  the  louclness,  number, 
and  pitch  of  the  accompanying  overtones  and  on  the  beats 
to  which  neighbouring  overtones  give  rise.  They  are  also  in 
part  the  outcome  of  association  with  the  instruments  which 
produce  the  tones ;  the  double  bass,  for  example,  suggesting 
the  sombre  immensity  of  low  tones,  the  trumpet  suggesting 
the  brightness  and  fineness  of  high  tones.  Yet  this  is  not  a 
complete  explanation  of  tone  character.  Sensations  of  tone 
(and  to  a  less  extent,  sensations  of  noise)  appear  to  contain 
a  certain  spatial  quality,  a  character  of  voluminousness, 
which  is  dependent  on  pitch.  This  tone  character  is 
perhaps  analogous  to  the  extensity  of  visual  and  tactile 
sensations.  Conjecture  may  relate  it  either  to  the  length  of 
the  sound  wave  or  to  the  number  of  simultaneously  excited 
hair  cells  within  the  cochlea. 

Variations  in  tone  character  make  it  extremely  difficult 
to  compare  the  intensities  of  tone  sensations  of  very  different 
pitch.  We  may  decide  without  trouble  that  c"  and  civ  are 
equally  or  differently  loud,  but  we  may  be  utterly  baffled  in 
comparing  the  loudness  of  c"  and  Gv 

Of  two  moderately  pitched  tone  stimuli  having  equal 
physical  intensity,  that  which  is  the  higher  in  pitch  will  give 
the  more  intense  sensation.  This  intenser  psychological 
effect  of  higher  tones  is  probably  closely  related  to  the 
special  tone  character  which  they  possess. 

[The  Intensity  of  Simultaneous  Tones. — Although  we  may 
with  fair  accuracy  compare  the  intensity  of  two  successive 
tone  sensations  which  are  nearly  alike  in  pitch,  we  should 
be  wrong  in  supposing  that  when  a  number  of  tones  are 
simultaneously  sounded,  the  intensity  of  the  whole  experi- 
ence depends  on  the  sum  of  the  intensities  of  the  different 
tone  sensations  which  are  present.  Provided  that  the  tone 
sensations  are  not  identical,  the  total  intensity  effect  is 
independent  of  their  number.  So  far,  indeed,  as  the 
component  sensations  are  concerned,  they  may  appear  in- 
dividually to  lose  in  loudness  when  excited  simultaneously. 


36  EXPERIMENTAL  PSYCHOLOGY 

This  is  particularly  the  case  when  high  and  low  tones 
are  sounded  simultaneously.  A  high  tone  appears  much 
louder  alone  than  when  sounding  with  a  low  tone.  Low 
tones  tend  to  suppress  or  to  obscure  accompanying  high 
tones  (exp.  26).  It  may  be  that  this  feature  is  again  the 
outcome  of  the  massive  tone  character  of  lower  tones ;  there 
may  be  some  obliterating  interaction  in  the  end  organ,  in 
the  peripheral  nerves,  or  in  the  auditory  centre.  At  present 
we  are  unable  to  decide.] 

The  Range  in  Pitch  of  Audible  Tones. — If  we  gradually 
raise  the  pitch  of  a  tone,  e.g.  by  shortening  the  length  of  a 
pipe,  the  tone  becomes  more  piercing,it  grows  finer  and  feebler, 
and  the  continuing  change  in  pitch  becomes  less  perceptible, 
until  finally  the  tone  ceases  altogether  to  be  heard.  The 
upper  limit  of  hearing  has  been  passed.  If  we  gradually 
lower  the  pitch  of  a  tone,  it  becomes  more  voluminous, 
intermittent,  and  noisy,  until  it  passes  into  a  series  of  pulses 
which  have  the  character  rather  of  thrusts  or  blows  on  the 
tympanic  membrane,  than  of  sounds. 

The  range  of  hearing  varies  in  different  animals.  In 
man  it  comprises  about  eleven  octaves,  but  the  limits  are 
ill-defined,  depending  on  the  age  and  practice  of  the  subject 
and  on  the  loudness  of  the  tone.  Whistles  (exp.  27), 
strings,  organ  pipes,  tuning-forks,  metal  rods,  and  toothed 
wheels  have  been  used  for  determining  the  range  of  hearing. 
Difference  tones  (page  42)  and  interruption  tones  (page  46) 
have  also  been  employed.  The  difficulty  in  determining  the 
lower  limit  consists  in  the  presence  of  overtones ;  the  first 
overtone  being  readily  mistaken  for  its  fundamental,  and  so 
leading  to  an  erroneous  result.  In  estimating  the  upper 
limit  of  hearing,  it  is  important  to  use  a  source  of  tone 
production  which  will  emit  a  tone  of  constant  pitch.  The 
notes  of  the  minute  whistle  (Galton's  whistle),  which  is 
usually  employed,  vary  considerably  in  pitch  according  to 
the  pressure  of  the  wind  blowing  it.  The  upper  limit  is 
also  influenced  very  sensibly  by  the  intensity  of  the  tone,  a 


AUDITORY  SENSATIONS  37 

feeble  tone  being  inaudible  while  a  louder  one  of  the  same 
pitch  may  yet  be  heard. 

The  lower  limit  of  hearing  is  a  tone  of  from  fifteen  to 
twenty  vibrations  per  second.  The  upper  limit  is  a  tone 
of  about  22,000  vibrations  per  second.  From  youth  on- 
wards the  range  narrows  at  both  ends,  becoming  reduced 
in  old  age  by  about  an  octave.  By  no  means  the  whole  of 
this  available  range  of  about  eleven  octaves  is  employed 
in  music.  The  notes  of  a  grand  pianoforte  extend  over 
a  distance  of  less  than  eight  octaves,  from  A2  (26'6  vibra- 
tions) to  cv  (4096  vibrations)  ;  the  organ  has  a  range  of  nine 
octaves  from  Cz  to  cvi. 

[The  Smallest  Perceptible  Difference  of  Pitch. — Some  of  the 
conditions  which  affect  our  ability  to  discriminate  between 
small  differences  will  be  alluded  to  hereafter  (Chap.  XIX.). 
The  smallest  interval  that  we  can  detect  between  two 
successive  tones  is  known  as  the  differential  threshold,  or  as 
the  threshold  of  discrimination,  for  successive  tones  (exp. 
120).  Over  the  middle  part  of  the  tone  range  it  is  fairly 
constant.  Practised  observers  are  just  able  to  detect  differ- 
ences in  the  case  of  the  following  pairs  of  tones  : — 64,  64'15  ; 
128,  128-16;  256,  256*23 ;  512,  512-25;  1024,  1024-22; 
2048,  2048-36.  Judgment  of  difference,  however,  is  easier 
than  one  of  direction  of  difference.  The  pairs  of  tones  just 
named  are  too  nearly  alike  for  the  observer  to  decide  which 
is  the  higher  or  the  lower,  even  though  he  may  be  able  to 
detect  a  difference  between  them.  Towards  the  upper  and 
lower  limits  of  the  range  of  audible  pitch,  discrimination 
becomes  less  delicate.  Indeed,  near  these  limits,  the  thres- 
hold of  discrimination  becomes  astonishingly  high.] 

Beats  and  Intertones. — If  two  tones  of  nearly  identical 
pitch  are  simultaneously  sounded,  "  beats "  occur,  the 
frequency  of  which  depends  on  the  vibration  difference 
of  the  tones  (exp.  29).  They  are  due  to  the  mutual  inter- 
ference of  the  two  tones.  Such  interference  may  be  proved 
by  mathematical  and  physical  methods  to  take  place  in  the 


38  EXPERIMENTAL  PSYCHOLOGY 

air  external  to  us.  If  two  series  of  pendular  waves  be 
drawn  on  paper  and  compounded  together,  the  one  contain- 
ing x,  the  other  x-\-y  vibrations  per  second,  it  will  be  found 
that  in  every  second  there  are  y  periods  of  rest,  i.e.  y 
interruptions  or  beats,  due  to  mutual  interference  of  the 
two  series.  We  shall  later  (page  53)  have  occasion  to 
discuss  whether  our  experience  of  beats  is  to  be  ascribed 
directly  to  this  external  interference  or  to  the  occurrence  of 
interference  within  the  ear.  For  the  present  this  need  not 
detain  us. 

Let  us  sound  together  two  tones  of  the  same  pitch,  c1 
(  =  256).  So  long  as  they  are  absolutely  in  unison  we  have 
a  perfectly  uniform  tone  sensation.  Now  let  one  of  these 
"  primes  "  be  slightly  raised  in  pitch,  and  we  shall  still  hear 
a  single  tone,  although  we  have  no  longer  a  continuous  tone 
sensation.  The  tone  regularly  swells  and  falls  in  loudness ; 
in  other  words,  it  "  beats."  The  beats  are  well  marked  and 
may  be  counted  until  the  pitch  of  the  mistimed  prime  begins 
to  exceed  264  vibrations.  If  we  raise  its  pitch  still  further, 
we  note  that  as  the  interruptions  become  more  rapid  they 
also  become  more  discontinuous,  more  "  beat-like."  The  rise 
and  fall  of  intensity  now  occur  with  great  suddenness.  The 
change  is  just  as  if  a  sea  of  waves  were  becoming  in- 
creasingly pointed,  and  less  rounded.  About  the  same 
time  unpleasant  tactile  sensations — due  to  tremor  of  the 
tympanic  membrane — begin  to  be  felt,  and  the  beats  begin 
to  be  accompanied  by  noise.  When  a  pitch  of  about  272 
vibrations  is  reached,  the  beats  are  most  thrust-like  and 
evident;  here  the  maximum  difference  between  a  rise  and 
fall  of  loudness  is  attained.  Beyond  this  point  the  noise 
and  the  unpleasant  tympanic  tremor  become  greater,  and 
the  individual  beats  seem  to  grow  smaller  and  flatter.  The 
most  disagreeable  effect  occurs  when  the  variable  tone 
reaches  about  284  vibrations.  From  now  onwards  a  com- 
plete separation  of  the  beats  becomes  more  and  more 
difficult.  They  begin  to  rattle  and  to  "  burr,"  and  ulti- 


AUDITORY  SENSATIONS  39 

mately  they  give  a  feeling  merely  of  roughness.  Even  at 
300,  interruptions  are  with  care  still  recognisable.  At  308 
all  trace  of  them  disappears  (exp.  30). 

Four  stages  may  thus  be  recognised,  as  the  frequency  of 
the  beats  is  increased.  In  the  first  stage  they  have  a 
surging,  in  the  second  a  thrusting,  and  in  the  third  a 
rattling  character ;  finally  they  fuse  and  pass  into  a  stage 
where  only  roughness  remains,  beyond  which  they  completely 
disappear.  But  these  four  stages,  although  recognisable  in 
the  middle  region  of  the  tone  range,  are  modified  in  the 
upper  and  lower  regions.  For  example,  the  rattle  met  with 
in  the  mid-region  becomes  a  chirp  in  the  upper  region ;  and 
the  beats  of  very  low  tones  disappear  without  reaching  the 
stage  of  roughness.  Beats  of  a  given  frequency  which  can 
still  be  counted  in  the  middle  tone  region  are  too  confused 
to  be  counted  in  the  lower.  In  the  example  given  above, 
the  beats  disappear  when  the  variable  tone  is  one  of  about 
308  vibrations  per  second,  i.e.  when  it  is  near  ef]°t  making  an 
interval  of  nearly  a  minor  third  with  c'.  On  the  other  hand, 
the  tone  C0  (64  vibrations)  just  ceases  to  beat  with  its  fifth 
#o — a  difference  of  32  vibrations.  Indeed,  the  limiting 
interval  for  the  beats  of  mistuned  primes  varies  according 
to  the  tone  region.  For  example,  cv  (4096  vibrations)  ceases 
to  give  even  a  feeling  of  roughness  with  tones  beyond  cv* — a 
difference  of  273  vibrations  (exp.  31). 

Other  things  being  equal,  the  beats  are  strongest  when 
the  two  tones  are  of  equal  loudness.  But,  as  we  have  seen, 
the  strength  of  beats  is  also  dependent  on  their  frequency, 
reaching  a  maximum  when  they  are  neither  very  rapid  nor 
very  slow ;  and  the  particular  frequency  which  yields  beats 
of  maximum  strength  varies  according  to  the  tone  region. 

If  two  near  tones,  e.g.  of  256  and  264  vibrations,  be 
sounded  together,  it  is  not  difficult  to  observe  that  the  beats 
arise  from  the  varying  intensity  of  the  sensation  of  a  single 
tone,  the  pitch  of  which  lies  roughly  midway  between  the 
pitch  of  the  two  primary  tones.  This  is  called  the 


40  EXPERIMENTAL  PSYCHOLOGY 

"  intertone."  As  the  interval  between  the  two  primary 
tones  is  increased,  the  intertone,  the  pitch  of  which  lies  at 
first  rather  nearer  the  lower  than  the  higher  of  the  tones, 
seems  to  rise  a  little  in  pitch,  and  it  becomes  less  evident. 
A  multiple  tone  impression  is  at  length  produced,  first  the 
higher,  then  the  lower,  of  the  two  primary  tones  emerging 
and  becoming  distinguishable  as  the  interval  is  increased. 
Careful  attention  shows  that  it  is  always  the  intertone 
which  carries  the  beats.  It  is  of  softer  character  than  the 
primary  tones,  and  is  localised  within  the  ear.  When  in 
the  above  example  the  mistuned  prime  reaches  300  vibra- 
tions, the  intertone  is  almost  inaudible  (exp.  32). 

[Sometimes  the  beating  tone  may  appear  to  vary,  not 
only  in  loudness,  but  in  pitch.  This  is  probably  due  to  the 
prominence  of  overtones  during  the  periods  of  relative 
silence,  and  is  thus  an  example  of  the  confusion  of  change 
of  timbre  with  change  of  pitch  (page  31). 

Beats  may  be  heard  when  the  two  tones  are  separately 
conducted,  one  to  one  ear,  the  other  to  the  other.  Careful 
experiments  have  been  made  upon  these  so-called  "  binaural " 
beats.  Two  electrically  driven  tuning-forks  were  used  as 
the  source  of  sound.  They  were  placed  one  in  each  of  two 
rooms  situated  on  either  side  of  a  central  room,  in  which 
the  observer  sat.  The  tones  were  led  to  his  ear  by  means  of 
sound-tight  tubes,  and  were  thus  completely  isolated  from 
one  another.  The  audibility  of  binaural  beats  was  held  to 
prove  that  our  experience  of  beats  is  of  central  origin,  due 
to  the  stimulation  of  the  auditory  centres  by  impulses 
travelling  up  the  auditory  nerves.  This  view  meets  with 
very  little  sympathy  at  the  present  day.  All  recent 
evidence  goes  to  show  that  when  two  tones  of  nearly 
identical  pitch  are  led  to  the  two  ears  separately,  each  tone 
passes  by  bone  conduction  to  the  opposite  ear,  where  by 
interference  with  the  other  tone  it  gives  rise  to  beats. 
Auscultation  of  the  skull  roof  by  means  of  a  specially 
constructed  microphone  has  definitely  proved  that  con- 


AUDITORY  SENSATIONS  41 

duction  occurs  in  this  way.  There  can  be  little  doubt 
that  uniaural  and  binaural  beats  have  the  same  (peripheral) 
origin. 

Beats  may  be  heard  when  the  primary  tones,  led  to 
separate  ears,  are  so  faint  that  each  when  sounding  alone  is 
inaudible.  We  shall  subsequently  see  (page  53)  that  con- 
duction from  ear  to  ear  is  a  possible  explanation  of  the 
audibility  of  binaural  beats  from  such  tones  of  subliminal 
intensity.] 

BIBLIOGRAPHY. 

H.  L.  F.  von  Helinholtz,  Sensations  of  Tone,  trans,  by  A.  J.  Ellis, 
London,  1S95.  C.  Stumpf,  Tonpsychologie,  Leipzig,  i.  ii.,  1883,  1890. 
K.  L.  Schafer,  "  Der  Gehorsinn,"  in  NageVs  Handbuch  d.  Physiol.  d. 
Menschen,  Braunschweig,  1905,  Hi.  476.  A.  Barth,  "  Zur  Lehre  yon  den 
Tonen  und  Gerauschen,"  Ztsch.  f.  Ohrcnhcilkundcn,  1887,  xvii.  81.  F. 
Kriiger,  "  Beobaclitungen  an  Zweikliingen,"  Philosoph.  Stnd.,  1900.  xvi. 
307,  568. 


CHAPTER    IV 
ON   AUDITORY   SENSATIONS1  (concluded) 

Combination  Tones. — When  two  different  tones,  not  too 
closely  similar  in  pitch,  are  sounded  simultaneously,  "  com- 
bination tones  "  are  heard  in  addition  to  the  two  primary 
tones.  They  are  localised  within  the  ear.  Combination 
tones  are  of  two  kinds,  "  summation  tones  "  and  "  difference 
tones."  The  former  can  only  be  heard  with  great  difficulty ; 
but  their  existence,  although  from  time  to  time  denied,  has 
been  unquestionably  proved  by  means  of  appropriate  re- 
sonators. 

The  pitch  of  the  summation  tone  corresponds  to  the 
sum  of  the  vibration  frequencies  of  the  two  primary  tones. 
The  pitch  of  the  difference  tone,  which  even  the  unprac- 
tised may  hear  without  difficulty  (exps.  33-35),  corresponds 
to  the  difference  of  these  frequencies.  Thus,  if  li  be  the 
pitch  of  the  higher  and  I  that  of  the  lower  primary  tone, 
h  —  l  will  be  the  pitch  of  the  difference  tone  produced  when 
h  and  /  are  simultaneously  sounded.  This  tone  is  some- 
times called  the  first  difference  tone,  Dp  because  there  are 
various  orders  of  difference  tones,  more  than  one  of  which 
may  be  simultaneously  present.  For  example  (exp.  36),  a 
second  difference  tone,  D2,  may  be  detected,  which  has  a 
pitch  2  /  —  h ;  it  may  be  looked  on  as  a  difference  tone 
between  the  lower  primary  and  the  first  difference  tone, 
I  — (h  —  l).  A  third  difference  tone,  D3,  may  also  occur, 
having  a  pitch  31  — 2  h, — a  difference  tone  between  the 
second  and  first  difference  tones.  Even  fourth  and  fifth 

1  See  footnote  at  the  beginning  of  Chapter  III. 
42 


AUDITORY  SENSATIONS  43 

orders  of  difference  tones,  D4  and  D5,  have  been  detected  by 
a  trained  observer.  Which  of  these  higher  orders  of  differ- 
ence tones  are  recognisable  is  said  to  depend  principally  on 
the  interval  between  the  primary  tones. 

[In  investigating  the  nature  of  combination  tones,  we 
have  first  to  ascertain  whether  they  are  of  subjective  or 
of  objective  origin.  That  is  to  say,  we  must  determine 
whether  pendular  vibrations,  corresponding  to  the  pitch  of 
the  combination  tones,  exist  in  the  air  when  the  two 
primary  tones  are  simultaneously  generated ;  or  whether 
these  sensations  of  combination  tones  are  of  our  own  making, 
devoid  of  any  external  physical  basis. 

To  test  the  independent  existence  of  pendular  waves  we 
have  recourse  to  resonators.  If  the  latter  picks  out  a  tone, 
its  objective  existence  is  proven.  Now  investigations  with 
appropriate  resonators  have  shown  that,  under  certain  con- 
ditions, combination  tones  are  of  objective  origin,  while, 
under  others,  they  are  mainly  or  solely  of  subjective  origin. 
The  particularly  favourable  condition  for  producing  objec- 
tive combination  tones  appears  to  be  this, — that  at  the 
moment  of  their  generation  the  two  primary  tones  must 
throw  one  and  the  same  enclosed  volume  of  air  simultan- 
eously into  vibration.1  For  example,  the  combination  tones 
produced  by  the  tones  of  a  siren  or  a  harmonium,  which  is 
blown  by  a  single  wind  chest,  are  objective.  But  if  two 
separate  bellows  be  used,  so  that  the  air  supply  for  each 
primary  tone  is  different,  then,  although  the  combination 
tones  are  hardly  less  audible  than  before,  they  are  very 
largely  of  subjective  origin ;  resonators  will  reinforce  them 
in  a  far  less  degree.  In  the  case  of  the  combination  tones 
of  tuning-forks,  practically  no  resonance  is  obtainable ;  the 
combination  tones  which  we  hear  must  be  almost  entirely, 
perhaps  entirely,  subjective. 

Objective  combination  tones  may  be  readily  explained 
by  mathematical  and  physical  considerations ;  whereas  the 

1  This  has  lately  been  disputed  by  Hermann  (oj).  ciL). 


44  EXPERIMENTAL  PSYCHOLOGY 

interpretation  of  subjective  combination  tones  has  always 
proved  a  far  more  difficult  problem.  From  recent  experi- 
ments, however,  it  appears  that  membranes,  resembling  the 
tympanic  membrane,  produce  objective  difference  tones 
when  thrown  into  vibration  by  two  simultaneous  tones.  If 
this  be  so,  we  have  an  obvious  explanation  of  the  origin  of 
combination  tones  within  the  ear.  The  few  cases  that  have 
been  recorded  of  patients,  devoid  of  tympanic  membrane  or 
of  malleus  and  incus,  who  are  yet  able  to  hear  subjective 
combination  tones,  perhaps  suggest  that  these  tones  may 
be  similarly  formed  at  the  fenestra  rotunda.  If  subjective 
combination  tones  really  arise  from  the  vibrations  of  such 
membranes,  it  becomes  intelligible  that  two  tones,  which 
are  so  high  in  pitch  as  to  appear  identical,  or  are  altogether 
beyond  the  limit  of  hearing,  nevertheless  give  audible  sub- 
jective difference  tones. 

In  order  to  avoid  complications,  it  is  essential  that  the 
tones,  used  for  producing  combination  tones,  be  as  pure  as 
possible.  Even  tuning-forks  are  not  devoid  of  overtones 
(page  30) ;  but  these  may  be  removed  by  the  use  of  inter- 
ference tubes.1  The  most  careful  work  upon  combination 
tones  has  been  done  under  conditions  where  the  sources  of 
sound  are  far  removed  from  the  listener,  and  the  sounds  are 
purified,  so  far  as  possible,  by  interference  apparatus  from 
any  accompanying  overtones  before  they  reach  his  ear. 

Experience  has  shown  that  weak  tones  are  far  more 
favourable  for  exact  observation  than  strong,  although 
earlier  observers  were  of  the  opposite  opinion.  The  primary 

1  One  method  of  "  interfering  "  with  a  tone  of  a  given  wave  length  is  to 
transmit  the  tone  through  a  bifurcating  tube.  The  two  arms  of  the  fork 
subsequently  reunite,  but  their  lengths  so  differ  that  the  sound  waves, 
passing  along  the  one  arm  are  at  the  point  of  reunion  in  opposite  phase 
to  those  passing  along  the  other.  Another  method  of  interference  is  to 
provide  the  conducting  tube,  somewhere  in  its  course,  with  one  or  more  side 
tubes  closed  at  their  ends.  The  side  tube  is  of  such  length  that  the  waves, 
after  entering  it  and  being  reflected  at  the  closed  end,  return  to  the  main 
tube  in  a  phase  opposite  to  that  of  the  newly  arriving  waves  which  have  not 
traversed  the  side  piece. 


AUDITORY  SENSATIONS  45 

tones  should  be  of  equal  intensity,  for  difference  tones 
become  less  evident  with  increasing  difference  of  intensity 
of  the  primary  tones.  Sometimes,  however,  a  different 
order  of  difference  tone  may  be  detected  when  the  relative 
intensity  of  the  primary  tones  is  changed. 

A  difference  tone  is  heard  with  far  more  difficulty  when 
its  pitch  lies  between  that  of  the  primary  tones.  A  simple 
calculation  will  show  that  if  the  primary  tones  bear  a 
simple  relation  to  one  another  (e.g.  1:2,  2:3,  3:4),  the 
various  orders  of  difference  tones  (page  42)  often  coincide ; 
and  it  is  under  these  circumstances  that  difference  tones 
are  best  heard. 

The  overtones  of  primary  tones  also  yield  combination 
tones  with  one  another.  Two  or  more  combination  tones 
and  overtones  may  thus  occur  at  the  same  moment,  which 
are  so  closely  similar  in  pitch  that  they  fuse  to  produce 
a  single  beating  intertone.  Such  beating  intertones  are 
formed  even  from  pure  (i.e.  overtone-free)  tones  when  con- 
sonant intervals  are  mistimed.  For  example,  a  tone  will 
beat  with  its  mistuned  octave.  If  the  first  overtone  be 
present,  such  beats  are  partly  to  be  ascribed  to  the  intertone 
formed  between  the  higher  primary  tone  and  the  first  over- 
tone of  the  lower  primary  tone.  But  even  when  the  latter 
overtone  is  removed  by  interference  tubes,  beats  are  still 
audible  owing  to  the  formation  of  a  difference  tone  of  nearly 
the  same  pitch  as  the  lower  primary  tone. 

Difference  tones  may  be  heard  when  the  primary  tones 
are  isolated  and  led  separately  to  the  two  ears,  or  when  one 
tone  is  led  to  the  ear  and  another  is  communicated  by  a 
timing-fork  applied  to  the  teeth  or  to  the  head.  Under 
such  circumstances,  conduction  of  both  primary  tones  to  one 
and  the  same  ear  unquestionably  takes  place. 

Konig  held  that  two  primary  tones  could  give  rise  to 
combination  tones  which  were  neither  summation  nor 
difference  tones.  At  one  time,  indeed,  he  denied  the  exist- 
ence of  difference  tones  altogether,  and  he  gave  the  name  of 


46  EXPERIMENTAL  PSYCHOLOGY 

"  beat  tones "  to  the  tones  which  he  obtained.  The  diffi- 
culties raised  by  Konig  were  the  inevitable  result  of  his 
employing  loudly  sounding  tuning-forks  to  produce  the 
primary  tones.  The  relatively  powerful  overtones  accom- 
panying Konig's  primary  tones,  and  the  complicated  series 
of  combination  tones  produced  by  them,  were  the  source  of 
the  many  difficulties  which  he  raised.  There  can  no  longer 
be  any  doubt  as  to  the  existence  of  difference  tones  and  the 
non-existence  of  beat  tones. 

The  formation  of  the  combination  tones  h  +  I  and  h  —  I, 
when  the  air  is  simultaneously  affected  by  vibrations  of 
periods  h  and  /,  may  be  attributed  to  the  periodic  disturb- 
ances arising  between  h  and  /.  We  may  thus  regard  objec- 
tive combination  tones  as  the  result  of  mutual  periodic  dis- 
turbances in  the  generation  of  the  primary  tones.] 

Variation  Tones. — Tones  of  like  pitch  and  of  like  forma- 
tion are  also  produced  when  I  is  no  longer  a  series  of  dis- 
turbing sound  waves,  but  consists  of  a  number  of  simple 
interruptions  applied  to  h.  Thus,  if  the  tone  h,  issuing  from 
a  siren,  be  interrupted  /  times  per  second  (by  the  closure  of 
alternate  groups  of  the  holes  in  its  rotating  disc),  the  tones 
h  -f  I  and  h  —  I  will  be  heard  in  addition  to  the  tone  h. 
These  tones  are  called  "  variation  tones,"  and  may  be 
strengthened  by  appropriate  resonators. 

Interruption  Tones. — According  to  the  most  recent  views, 
either  of  these  two  variation  tones  may  produce  a  difference 
tone  with  the  primary  tone.  At  all  events,  a  tone  may  un- 
doubtedly be  heard,  under  suitable  conditions,  which  has  the 
pitch  l(  =  li  +  l  —  h,orh-  h  —  l).  This  latter  tone  is  called  the 
"  interruption  tone,"  its  pitch  coinciding  with  the  number  of 
interruptions  given  per  second  to  the  primary  tone.  By 
means  of  an  appropriately  attuned  resonator,  it  also  has 
been  proved  to  have  an  objective  existence. 

[The  interruption  tone  is  best  heard  when  its  pitch  is 
considerably  different  from  that  of  the  primary  tone,  and 
when  the  primary  tone  is  so  high  that  the  variation  tones 


AUDITORY  SENSATIONS  47 

differ  little  from  it,  and  thus  are  feebly  audible.  On  the 
other  hand,  the  variation  tones  are  best  heard  when  the 
primary  tone  is  so  low  that  the  still  lower  interruption  tone 
is  inaudible.  Like  ordinary  difference  tones,  the  inter- 
ruption tone  is  localised  within  the  ear. 

Various  methods  have  been  utilised  in  order  to  produce 
variation  and  interruption  tones.  If  a  siren  be  employed, 
the  disc  of  which  contains  three  series  each  of  ninety-six 
holes,  and  if  in  the  first  series  every  alternate  group  of  four 
holes  be  shut,  in  the  second  series  every  alternate  group  of 
three  holes  be  shut,  and  in  the  third  series  every  alternate 
pair  of  holes  be  shut, — we  shall  clearly  be  able  to  demon- 
strate three  different  interruption  tones  having  vibration 
frequencies  in  the  ratio  1  :  |-  :  2.  For  in  the  first  series 
there  are  twelve  interruptions,  in  the  second  sixteen, 
and  in  the  third  twenty-four  interruptions  per  revolution. 
As  there  are  ninety-six  holes,  the  vibration  frequency 
of  the  highest  of  the  three  interruption  tones  must  be 
one-quarter  (i.e.  two  octaves  below)  that  of  the  primary 
tone. 

An  interruption  tone  may  also  be  obtained  when  a  card 
is  made  to  press  against  a  rapidly  revolving  toothed  wheel, 
the  teeth  of  which  are  at  regular  intervals  filled  up  or 
removed.  The  pitch  of  the  primary  tone  is  determined  by 
the  total  number  of  both  closed  and  open  teeth.  The 
number  of  interruptions  in  the  teeth  determines  the  pitch 
of  the  interruption  tone.  An  interruption  tone  is  also 
produced  when  the  vibrations  communicated  by  a  tuning- 
fork  to  the  air  are  periodically  interrupted  by  a  rotating  disc 
provided  with  alternate  closed  and  open  sectors.] 

The  Relation  of  Tones. — If  various  pairs  of  successively 
sounding  tones  be  compared  (exp.  37),  a  closer  relation  and 
an  easier  transition  will  be  found  between  the  members  of 
some  pairs  than  between  those  of  others.  The  tone  nearest 
related  to  a  given  tone  will  prove  to  be  the  octave  or  any 
multiple  of  the  octave.  Even  practised  musicians,  when 


48  EXPERIMENTAL  PSYCHOLOGY 

asked  to  identify  a  given  tone,  are  apt  to  confuse  octave 
tones  with  one  another. 

One  might  at  first  feel  disposed  to  express  this  relation 
throughout  the  tone  range  by  means  of  a  spiral,  along  which 
the  tones  are  marked  off  in  order,  and  successive  octave 
tones  are  so  placed  that  they  overlie  one  another.  But 
such  a  diagram  would  fail  to  represent  the  facts  at  all 
accurately,  inasmuch  as  the  next  most  nearly  related 
members  are  found  to  be  a  tone  and  its  fifth  above,  which 
would  lie  on  such  a  spiral  at  a  point  furthest  removed  from 
the  lower  tone.  After  the  fifth,  the  fourth  is  the  next 
closely  related  interval.  Then  follow  the  major  and  minor 
thirds  and  the  sixths. 

The  confusion  of  a  tone  with  its  octave  may  be  also 
demonstrated  by  sounding  the  two  tones  simultaneously 
and  asking  a  listener  to  report  whether  one  or  more  tones 
are  present.  If  the  octave  tone  be  weak  in  comparison 
with  the  lower,  it  is  unnoticed,  except  in  so  far  as  it  alters 
the  timbre  and  the  apparent  pitch  of  the  lower  tone  (page 
31).  Even  when  the  two  tones  are  of  equal  strength,  a 
trained  musician  will  sometimes  return  wrong  judgments. 
Unmusical  people  experience  a  like  difficulty  of  analysis  in 
the  case  of  other  less  closely  related  pairs  of  tones  (exp.  38), 
the  percentage  of  their  wrong  answers  being  approximately 
as  follows  :  — 

Thirds  and  Seconds  and 

Octave.  Fifth.  Fourth. 


75  50  30  20  10 

There  is  hence  a  very  strong  tendency  for  octaves,  and  a 
diminishing  tendency  for  fifths,  fourths,  thirds,  and  sixths,  to 
produce  an  apparently  single  tone  sensation.  This  blending 
or  fusion  of  simultaneous  tones  corresponds  in  degree 
precisely  with  the  recognised  order  of  "  consonance  "  or 
agreeableness  of  the  intervals  in  music.  The  octave  is  the 
most  perfect  consonance,  of  course  excepting  unison.  Then 


AUDITORY  SENSATIONS  49 

follows  the  fifth,  next  the  fourth,  and  lastly  the  major  and 
minor  thirds  and  sixths  (page  29). 

The  remaining  intervals  within  the  octave,  e.g.  seconds 
and  sevenths,  and  the  corresponding  intervals  beyond  the 
octaves,  e.g.  ninths  and  fifteenths,  are  termed  "  dissonances." 
The  dual  nature  of  such  simultaneously  sounding  tones  can 
be  recognised  without  difficulty  even  by  unmusical  observers ; 
they  show  hardly  any  tendency  to  fusion.  Want  of  fusion 
and  dissonance  also  occurs  in  a  most  marked  degree  when 
consonant  intervals  are  appreciably  mistuned. 

The  Absolute  Determination  of  Pitch. — We  may  be  able 
to  determine  the  pitch  of  a  given  tone  by  observing  the 
interval  between  it  and  a  previously  given  tone  the  pitch  of 
which  is  already  known.  Or  we  may  be  able  to  determine 
the  pitch  absolutely,  without  reference  to  any  other  tone. 
The  absolute  determination  of  pitch  is  then  dependent  on  a 
close  association  between  each  tone  and  its  alphabetical 
name.  When  the  tone  is  sounded,  its  name  immediately 
appears  in  consciousness. 

This  close  association  of  tones  with  their  names  is 
acquired  in  much  the  same  way  as  the  association  of  colours 
with  their  names,  or  as  the  association  of  different  shades  of 
timbre  with  the  different  voices  or  instruments  that  produce 
them.  It  is  often  learnt  quite  early  in  life,  but  it  can,  at 
least  sometimes,  be  acquired  later,  and  can  be  much 
improved  by  practice.  There  are  great  individual  differ- 
ences in  the  scope  of  absolute  pitch  determination.  Of 
course,  in  the  majority  of  persons  it  is  entirely  lacking. 
Some  can  only  name  a  single  tone,  others  can  identify  a 
narrow  or  a  wide  range  of  tones.  In  some  persons  the 
name  of  the  note  is  not  suggested  directly.  They  need  the 
intermediate  visual  imagery  of  the  alphabet  or  of  the  keys 
of  the  pianoforte,  or  visual  or  kinsesthetic  imagery  of  the 
hands  or  fingers  moving  upon  the  instrument. 

Closely  allied  to,  but  by  no   means  always  correlated 
with  this  talent  is  the  power  to  produce  a  tone  whenever 
4 


50  EXPERIMENTAL  PSYCHOLOGY 

its  name  is  given.  The  exact  pitch  of  the  tone  thus 
produced  depends  on  the  pitch  of  the  instrument  at  which 
the  individual  has  been  musically  educated.  So,  too,  when 
asked  to  name  a  sounded  tone,  he  is  most  accurate  in  his 
replies  to  the  tones  of  the  particular  instrument  to  which 
he  has  been  most  accustomed.  Inaccuracies  occur  when 
the  tone  is  of  unusual  timbre  or  intensity,  or  when  the 
subject  is  excited,  depressed,  or  fatigued. 

It  is  far  easier  for  a  person  so  gifted  to  say  that  a  given 
tone  is  c,  d,  or  e,  than  for  him  to  say  which  particular  c,  d, 
or  e  is  sounded.  He  may  never  confuse  d'  with  c  or  with  e, 
but  he  always  finds  it  difficult  to  distinguish  d'  from  d" 
or  from  d°.  Occasionally  the  confusion  occurs  not  only 
between  octaves  but  between  fifths,  the  next  consonant 
interval. 

We  have  no  definite  knowledge  of  the  methods  by  which 
the  association  of  tones  with  their  names  is  acquired.  It  is, 
however,  noteworthy  that  young  children  (and  birds),  when 
once  they  have  been  taught  a  short  musical  phrase,  tend  to 
repeat  it  without  any  subsequent  alteration  in  pitch.  This 
tendency  is  obviously  favourable  for  a  sensibility  to  absolute 
pitch,  but  it  is  soon  discouraged,  owing  to  the  fact  that  they 
generally  hear  the  same  melody  repeated  in  different  keys. 
Sensibility  for  intervals  replaces  sensibility  for  absolute 
pitch. 

The  Cochlea. — So  far,  in  speaking  of  auditory  stimuli,  we 
have  traced  their  course  only  so  far  as  the  fenestra  ovalis  of 
the  middle  ear.  The  sound  vibrations  now  pass  into  the 
fluid  perilymph  and  travel  to  the  end  organs  of  the  inner 
ear. 

In  the  higher  mammals  the  end  organs  of  the  saccule, 
the  utricle,  and  the  semicircular  canals  are  predominantly, 
if  not  wholly,  concerned  with  functions  other  than  hearing 
(Chap.  V.).  In  them  the  cochlea  has  reached  its  greatest 
complexity. 

There  are  serious  experimental  difficulties  in  the  removal 


AUDITORY  SENSATIONS  51 

of  one  or  other  of  these  end  organs  from  the  vertebrate 
inner  ear.  Even  when  the  operation  has  been  successful, 
errors  may  arise,  and  have  actually  arisen,  owing  to  the  use 
of  auditory  tests  of  such  loudness  that  the  animal  becomes 
aware  of  the  vibrations  by  the  sense  of  touch  instead  of  by 
hearing.  Yet  such  evidence  as  has  been  obtained  by  experi- 
mental removal  of  the  cochlea  is  in  favour  of  the  view 
already  mentioned,  that  in  the  higher  mammals  it  is 
pre-eminently  the  organ  of  hearing. 

Helmholtz1  s  Theory  of  Hearing. — Helmholtz,  observing 
that  we  have  the  power  possessed  by  resonators  of  analysing 
mixtures  of  tones  (complex  periodic  waves)  into  their  simpler 
tonal  constituents  (component  pendular  waves),  constructed 
a  theory  of  hearing  on  the  analogy  of  physical  resonance 
(page  22).  He  supposed  that  the  cochlea  contains  a  vast 
number  of  differently  resonating  structures,  each  of  which 
is  attuned  to  a  tone  of  definite  pitch  and  is  thrown  into 
sympathetic  vibration  when  the  appropriate  tone  reaches 
the  cochlea.  The  vibrations,  passing  up  the  perilymph  of 
the  scala  vestibuli  and  down  the  scala  tympani,  are  trans- 
mitted across  the  cochlear  canal  in  their  course.  A  given 
tone  will  thus  cause  one  of  the  resonators  of  the  cochlea  to 
vibrate,  leaving  others  untouched ;  and  each  of  these  others 
will,  in  turn,  vibrate  when  a  suitable  tone  reaches  them. 
According  to  this  conception,  the  cochlea  resembles  in 
construction  the  pianoforte ;  any  string  of  which,  as  is  well 
known,  can  be  thrown  into  vibration,  if  the  tone  to  which  it 
is  attuned  be  sounded  before  it.  After  some  hesitation, 
Helmholtz  concluded  that  the  fibres  of  the  basilar  membrane, 
which  number  about  twenty-four  thousand  in  the  human 
cochlea,  are  the  resonant  structures  comparable  to  the 
strings  of  the  pianoforte. 

This  hypothesis  has  met  with  much  support  in  different 
directions,  despite  certain  difficulties  which  we  shall  later 
have  cause  to  discuss.  If  the  evidence  of  an  undoubtedly 
difficult  experiment  can  be  accepted,  dogs  become  deaf  to 


52  EXPERIMENTAL  PSYCHOLOGY 

high  tones  when  the  lowest  whorl  of  the  cochlea  is  destroyed, 
and  to  low  tones  when  the  highest  whorl  of  the  cochlea  is 
destroyed.  It  has  also  been  found  that  the  lowest  whorl  of 
the  cochlea  was  affected  in  the  case  of  a  deaf  boilermaker, 
the  noise  of  boilermaking  being  conducive  to  deafness, 
especially  to  high  tones.  Now  these  are  just  the  results  we 
should  expect,  were  Helmholtz's  theory  correct;  for  the 
basilar  fibres  increase  in  length  from  the  base  to  the  apex 
of  the  cochlea,  and  it  may  be  presumed  that  the  shortest 
fibres  resonate  to  the  highest  tones. 

Moreover,  careful  examination  has  revealed  that  persons 
may  be  deaf  to  a  few  isolated  tones  only,  or  may  hear  only 
in  very  limited  regions,  throughout  the  tone  range.  These 
"  tone  gaps "  and  "  islands  of  hearing,"  as  they  may  be 
termed,  receive  a  ready  explanation  at  the  hands  of 
Helmholtz's  hypothesis.  It  is  easily  imaginable  that  certain 
basilar  fibres  or  groups  of  basilar  fibres  have  ceased  to 
vibrate  properly,  or  that  the  hair  cells  and  nerves,  which  the 
vibration  of  these  fibres  should  stimulate,  no  longer  behave 
as  they  ought. 

Conditions  have  been  described  in  which  a  single  tone 
produces  a  double  tone  sensation  in  one  ear  ("  uniaural 
diplacusis "),  or  is  judged  by  one  ear  to  have  a  pitch  con- 
siderably different  from  that  which  it  appears  to  have  when 
heard  by  the  other  ("  binaural  diplacusis  ").  Such  anomalies 
may  perhaps  be  attributed  to  the  disordered  function  of 
certain  basilar  fibres,  or  of  the  hair  cells  and  nerve  fibres 
supported  by  them. 

Extensions  of  Helmlwltzs  Theory. — If  the  cochlea  really 
contains  a  mass  of  variously  attuned  fibres,  the  principles  of 
resonance  require  that  a  given  tone  will  throw  into  vibration 
not  only  the  fibre  to  which  it  is  accurately  attuned,  but  also 
neighbouring  fibres  which  are  attuned  to  tones  very  slightly 
higher  or  lower  in  pitch  (page  23).  These  latter  fibres  will 
vibrate  with  diminishing  amplitude  according  to  their  dis- 
tance from  the  fibre  to  which  the  tone  exactly  corresponds. 


AUDITORY  SENSATIONS  53 

In  consequence,  the  effect  of  any  given  tone  on  the  basilar 
membrane  may  be  graphically  represented  by  a  wave,  the 
apex  of  the  wave  coinciding  with  the  most  powerfully 
vibrating,  i.e.  the  exactly  attuned,  fibre ;  on  either  side  of 
which  the  fibres  vibrate  with  less  and  less  force  in  pro- 
portion to  their  remoteness.  According  to  this  conception, 
the  pitch  of  a  tone  is  determined  by  the  position  of  the  apex 
of  the  imaginary  wave.  Now,  if  two  tones  of  nearly 
identical  pitch  be  simultaneously  sounded,  these  two  waves 
will  very  closely  overlap,  and  will  by  summation  yield  a 
single  wave  which  has  an  apex  somewhere  intermediate 
between  the  apices  of  the  component  waves. 

By  this  extension  of  the  resonance  hypothesis,  intertones 
(page  40)  become  readily  explicable.  It  is  obvious  that,  as 
the  interval  between  two  nearly  identical  tones  becomes 
greater,  and  as  the  overlapping  of  the  two  waves  diminishes 
in  extent,  the  apex  of  each  individual  wave  must  become 
prominent,  the  two  tones  must  become  separately  audible, 
and  the  intertone  must  finally  vanish.  The  beating  char- 
acter of  the  intertone  is  ascribable  to  the  interferent  action 
of  the  two  series  of  sound  waves  on  the  same  fibres. 

On  the  same  grounds,  we  can  explain  the  audibility  of 
binaural  beats  produced  by  tones  of  subliminal  intensity 
(page  41).  The  tones  are  conducted  to  one  and  the  same  ear 
by  bone  conduction ;  their  wave  effects  on  the  basilar  fibres 
overlap  and  strengthen  one  another  so  as  to  produce  an 
audible  beating  intertone.  A  nearly  liminal  sensation,  of 
course,  becomes  more  effective  when  intermittent. 

Helmholtz's  hypothesis  implies  that  (so  far,  at  least,  as 
tone  sensations  are  concerned)  the  apparatus  of  the  inner 
ear  has  no  concern  with  the  actual  movements  of  the 
membranes  and  ossicles  and  of  the  external  air.  The 
cochlea  merely  resolves  these  vibrations  into  their  pendular 
constituents,  utterly  disregarding  the  complex  unresolved 
movements  of  the  conducting  media. 

If  this   be   true,  two  or  more  simultaneously  sounding 


54  EXPERIMENTAL  PSYCHOLOGY 

tones  must  produce  the  same  tonal  experience,  whatever  be 
the  respective  phases  in  which  their  waves  are  combined. 
The  ear  must  take  no  account  of  the  very  different  move- 
ments of  the  air,  ossicles,  membranes,  and  perilymph  which 
result  from  combining  the  same  two  series  of  waves  in 
varying  phases  relatively  to  one  another.  Many  investiga- 
tions have  been  made  in  order  to  determine  whether  the 
cochlea  is  affected  by  these  differences  of  phase,  or  merely 
resolves  the  periodic  curve  into  its  pendular  components. 
It  is  impossible  here  to  review  the  experimental  difficulties 
and  the  errors  which  have  beset  the  investigation  of  this 
important  question.  The  broad  conclusion  of  the  majority 
of  reliable  workers  need  only  be  quoted, — that  differences 
in  the  phase  of  sound  waves  make  no  appreciable  change 
in  the  experience  of  tone  sensations.1 

Now  this  view,  that  our  experience  of  tones  and  beats 
is  determined  solely  by  the  analytical  action  of  resonators 
within  the  inner  ear,  has  been  opposed  by  many  physicists 
and  physiologists,  who  have  believed  that  any  regular 
interruptions  in  a  tone  stimulus,  if  sufficiently  rapid,  can 
develop  a  sensation  of  tone.  There  are  some  facts  which 
at  first  sight  appear  to  tell  in  favour  of  this  belief.  For 
example,  if  a  tone  is  interrupted  sufficiently  rapidly,  an  inter- 
ruption tone  (page  46)  is  produced.  Again,  if  the  interval 
between  two  tones  is  not  too  close,  difference  tones  are 
produced.  In  each  of  these  cases  the  pitch  of  the  tone 
depends  on  the  frequency  of  the  interruption.  Further, 
Kb'nig,  as  we  have  seen  (page  46),  went  so  far  as  to  name 
difference  tones  "  beat  tones."  He  supposed  that  a  number 
of  sufficiently  rapid  interruptions  in  the  movements  of  the 
tympanic  membrane  must  give  rise  to  a  definite  tone 
sensation;  in  other  words,  that  within  certain  limits  of 
rapidity  every  kind  of  periodic  vibration  can  develop  a 
tone  sensation. 

1  The  effect  of  binaural  phase  difference  on  sound  localisation  is  discussed 
in  Chapter  XXI. 


AUDITORY  SENSATIONS  55 

But  such  a  view  is  irreconcilable  with  Helmholtz's  hypo- 
thesis that  only  pendular  vibrations  can  cause  tone  sensations. 
And  its  improbability  is  shown  by  the  following  considera- 
tions. If  beats  really  pass  over  into  difference  tones,  the 
difference  tone  ought  not  to  be  audible  until  the  beats  have 
disappeared.  In  point  of  fact,  the  presence  of  beats  may 
still  be  felt  when  high  tones  are  simultaneously  sounded, 
whicli  give  at  the  same  time  an  obvious  difference  tone. 
Moreover,  if  the  first  difference  tone  arises  from  rapid  beats, 
the  higher  orders  of  difference  tones  and  the  summation 
tone  become  wholly  inexplicable.  As  regards  the  inter- 
ruption tone,  a  different  and  more  satisfactory  interpretation 
has  been  already  given  (page  46).  There  is  therefore  no 
reason,  at  present,  to  discredit  Helmholtz's  view  that  only 
pendular  vibrations  can  give  rise  to  a  tone  sensation. 

At  first  believing  that  the  sensations  of  noise  and  tone 
arose  from  separate  end  organs,  Helmholtz  thought  that 
the  saccule  was  the  seat  of  noise  sensations,  but  later  he 
abandoned  this  idea.  Certainly  the  psychological  characters 
of  noise  and  tone  are  very  different  (page  27),  but  that  is  not 
a  sufficient  reason  for  supposing  that  the  cochlea  is  concerned 
solely  with  developing  tone  sensations.  Were  this  so,  we 
should  expect  that  pathological  conditions  would  arise, 
causing  deafness  to  tones,  while  noises  could  still  be  heard, 
— or  vice  versd.  Such  conditions,  however,  have  never  been 
recorded. 

We  have  already  explained  (page  53)  that  according  to 
Helmholtz's  theory  the  pitch  of  a  tone  sensation  must 
depend  on  the  position  of  the  most  strongly  stimulated 
basilar  fibre.  Now,  let  us  add  to  this  hypothesis  that  a 
sensation  of  noise  results,  when  one  fibre  does  not  vibrate 
more  strongly  than  the  rest.  Then  we  can  account  for  the 
occurrence  of  noise,  whether  the  stimulus  be  the  simul- 
taneous sounding  of  many  nearly  identical  tones,  an 
explosion,  or  a  blow  on  the  ears.  For  in  all  three  con- 
ditions a  considerable  part  of  the  basilar  membrane  is 


56  EXPERIMENTAL  PSYCHOLOGY 

thrown  into  uniform  vibration,  and  there  is  no  well- 
defined  point  of  maximal  stimulation.  Such  a  point 
may  conceivably  be  lacking,  when  a  tone  stimulus  lasts 
for  an  exceedingly  brief  time  (page  26);  so  that  here 
again  we  meet  with  a  reason  for  the  development  of  a 
noise  sensation. 

[Theories  of  Consonance. — We  may  now  take  up  the 
deferred  problem  of  the  basis  of  harmony.  According  to 
Helmholtz,  the  relation  of  two  consecutive  tones  to  one 
another  depends  on  the  degree  of  coincidence  or  of  dis- 
similarity of  their  overtones.  A  given  tone  will  therefore 
be  most  nearly  akin  to  the  octave  tone  following  it,  because 
all  the  harmonic  overtones  of  the  latter  will  occur  in  those 
of  the  former.  In  the  case  of  the  fifth,  the  one  tone — which 
we  may  denote  by  2n — will  produce  the  overtones  4n,  Qn, 
8n,  Wn,  12n,  etc. ;  while  the  other  tone,  3n,  will  produce  the 
overtones  6n,  9n,  I2n,  etc.  Here  the  tonal  relation  is  less 
close  than  in  the  case  of  the  octave,  alternate  overtones  of 
the  tone  3n  being  contained  in  those  of  the  tone  2n.  Still 
fewer  overtones  will  be  shared  by  the  tones  of  a  fourth,  and 
so  on  for  the  various  intervals. 

This  principle,  that  the  relation  between  two  different 
tone  sensations  depends  on  the  presence  of  identical  stimuli, 
may  be  likewise  traced  in  the  further  elaboration  of  Helm- 
holtz's  theory  of  hearing  proposed  by  Ebbinghaus.  He 
suggested  that  a  given  tone  of  n  vibrations  per  second 
affects  not  only  its  properly  attuned  fibre  of  the  basilar 
membrane,  but  also  those  fibres  attuned  to  tones  of  f ,  f ,  f , 
.  .  .  vibrations  per  second,  causing  them  to  vibrate  in  half, 
thirds,  quarters  .  .  .  from  1,  2,  3  .  .  .  nodal  points  respect- 
ively. According  to  this  view,  the  relation  between  two 
successive  tones,  even  when  free  from  overtones,  depends  on 
the  number  of  basilar  fibres  that  the  two  tones  stimulate 
in  common. 

There  can  be  little  doubt  that  musical  intervals  were 
originally  chosen  from  successively  sounding  tones.  Primi- 


AUDITORY  SENSATIONS  57 

tivo  men  played  and  sang  in  unison  long  before  they 
practised  polyphonic,  i.e.  many  part,  music.  When,  however, 
different  tones  began  to  be  simultaneously  sounded,  new 
aesthetic  effects,  literal  "  consonance "  and  "  dissonance," 
were  produced, — due  in  Helmholtz's  opinion  to  the  presence 
or  absence  of  beats.  Certainly  the  absolute  consonances, 
unison  and  octave,  yield  no  beats.  Fifths  only  give  rise  to 
beats  when  the  higher  overtones  are  exceptionally  loud  and 
beat  with  one  another.  Fourths,  thirds,  and  sixths  produce 
beats  more  easily.  Indeed,  the  minor  sixth  (5  :  8)  is  on 
the  border  line  between  consonance  and  dissonance,  the 
second  overtone  (15)  of  the  lower  tone  beating  audibly  with 
the  first  overtone  (16)  of  the  higher.  In  dissonant  intervals, 
the  beats  obtrude  still  more.  According  to  Helmholtz, 
our  judgment  of  consonance  or  dissonance  in  the  case  of 
simultaneous  tones  depends  on  the  relatively  continu- 
ous or  interrupted  (i.e.  beating)  character  of  our  tonal 
experience. 

If  beating  overtones  determine  the  dissonance  of  intervals, 
it  is  evident  that  dissonance  should  be  much  less  in  the  case 
of  intervals  which  are  produced  by  tones  freed  from  over- 
tones. Helmholtz  and  many  other  observers  have  declared 
this  to  be  true,  although  later  experiments,  of  a  somewhat 
unsatisfactory  nature,  have  been  brought  forward  to  con- 
tradict it. 

More  recently  attention  has  been  drawn  to  the  effect  of 
mistimed  consonances  on  the  pitch  of  the  difference  tones 
of  various  orders.  It  has  been  shown  that  in  such  dissonant 
intervals  beats  arise  from  the  intertones  which  are  formed 
between  neighbouring  difference  tones  and  (especially  the 
lower  of)  the  primary  tones,  while  in  the  case  of  consonant 
intervals  the  difference  tones  are  free  from  beats  and 
owing  to  their  coincidence  are  much  fewer  in  number 
(page  45). 

Let  us  repeat,  however,  that  pure  tones  are  a  philo- 
sophical fiction.  For  every  tone  is  accompanied  by  one  or 


58  EXPERIMENTAL  PSYCHOLOGY 

more  overtones;  and  even  when  they  are  obliterated  by 
interference,  they  may  conceivably  be  re-formed  within  the 
ear  before  the  end  organ  is  reached.  When  we  judge  that 
a  tone  is  pure,  we  do  so  because  we  are  incapable  of  analys- 
ing the  really  complex  pattern  of  vibration  received  at  the 
cochlea :  complex,  because  of  the  inevitable  overtones  which 
are  simultaneously,  be  it  ever  so  feebly,  stimulating  the 
appropriate  end  organs.  Throughout  our  life,  no  tones  are 
more  closely  associated  with  any  given  musical  tone  n,  than 
one  or  more  of  its  accompanying  overtones  2n,  3n,  4ft, 
5ft,  Qn,  unanalysed  though  they  be.  And  these  are  the 
very  tones  that  form  with  one  another  the  most  consonant 
intervals.  Thus  harmony  is  seen  to  have  a  natural  basis  of 
association. 

We  have  already  pointed  out  (page  48)  that  simultan- 
eously sounding  tones  of  consonant  intervals  are  with 
difficulty  analysable.  They  blend  smoothly  with  one 
another  and  fuse  more  or  less  perfectly,  so  that  the  result- 
ing experience  is  or  resembles  that  of  a  single  tone  sensa- 
tion. According  to  Stumpf,  this  fusion  is  the  psychologically 
irreducible  criterion  of  consonance  and  dissonance.  For 
him  neither  beats  nor  the  relations  of  overtones  constitute 
a  satisfactory  basis  of  harmony.  But  it  is  impossible  here 
to  enumerate  or  to  discuss  his  objections.] 

Criticism  of  Helmholtzs  Theory  of  Hearing. — The  difficulty 
of  Helmholtz's  theory  of  hearing  is  mainly  one  of  concep- 
tion. The  24,000  basilar  fibres  range  in  length  only  from 
about  0*04  to  048  mm.  Nevertheless  they  are  expected  to 
vibrate  to  tones  varying  between  15  vibrations  and  over 
20,000  vibrations  per  second.  Compensation  for  such  small 
differences  in  length  of  fibre  is  only  possible  by  almost 
inconceivably  great  differences  in  loading.  Again,  it  is 
hard  to  believe  that  the  basilar  fibres  are  free  to  vibrate 
as  stretched  strings.  They  are  lodged  in  a  homogeneous 
matrix,  supported  by  connective  tissue.  Nor,  in  any  event, 
can  they  vibrate  along  their  entire  length,  carrying  as  they 


AUDITORY  SENSATIONS  59 

do  the  rods  of  Corti  and  a  small  vein  beneath  the  tunnel. 
The  pars  pectinata  appears  to  be  the  only  freely  vibratile 
portion,  but  its  variations  in  length,  in  different  regions  of 
the  cochlea,  are  still  less  than  the  variations  in  length  of 
the  entire  fibre. 

We  may,  perhaps,  ask  if  the  power  of  analysing  mixed 
tones  into  their  constituents,  closely  as  it  resembles  the 
phenomenon  of  physical  resonance,  may  not  have  a  far  more 
obscure  physiological  basis.  Whether  the  living  protoplasm 
of  the  hair  cells  themselves  can  have  this  analytical  pro- 
perty, and  if  so  what  is  its  nature,  we  are  quite  unable  to 
say.  But  we  may  bear  in  mind  that  the  hairs  of  hair  cells 
in  certain  invertebrata  have  been  observed  to  vibrate 
selectively  to  special  tones;  that  the  cochlea  is  so  con- 
structed as  to  permit  of  the  ready  conduction  of  vibrations 
to  the  hair  cells ;  that  the  hairs  vary  in  length  in  different 
regions  of  the  cochlea ;  and  that  their  vibrations  are  doubt- 
less damped  by  the  overlying  tectorial  membrane. 

Rutherford's  Theory.  —  Eutherf ord  suggested  that  the 
peripheral  end  organs  of  the  ear  act  merely  as  a  telephone 
plate  which  receives  various  patterns  of  vibrations  and 
transmits  them  by  the  auditory  nerves  to  the  brain,  where 
analysis  actually  takes  place.  Such  an  hypothesis — unsatis- 
factory, if  only  because  it  pushes  explanations  still  further 
into  the  unknown — serves  to  remind  us  that  the  peripheral 
mechanism  is  not  the  sole  determinant  of  auditory  sensa- 
tion. Yet  in  hearing  as  in  vision,  the  task  of  deciding  how 
far  the  process  of  sensation  is  due  to  peripheral  and  how  far 
to  central  factors,  lies  confessedly  beyond  us. 

Ewald's  Theory. — The  difficulty  of  conceiving  that  the 
basilar  membrane  behaves  like  a  series  of  resonating  struc- 
tures, led  Ewald  to  study  the  actual  movements  of  non- 
living membranes  in  response  to  sound  vibrations.  In  his 
earlier  experiments  he  employed  an  elastic  membrane, 
which  was  stretched  loosely  in  its  longitudinal  direction  on 
a  wooden  frame,  and  in  its  transverse  direction  was  either 


60  EXPERIMENTAL  PSYCHOLOGY 

held  more  tensely  or  more  often  was  not  stretched  at  all. 
A  vibrating  fork,  when  pressed  against  the  end  of  such  a 
membrane,  was  found  to  form  upon  it  a  series  of  waves, 
which  were  visible  as  dark  transverse  streaks  or  nodal  lines, 
and  could  be  photographed  under  suitable  conditions. 
These  streaks  or  lines  were  termed  by  Ewald  "  sound  pic- 
tures." Pie  found  that  the  number  of,  and  the  distance 
between,  successive  transverse  lines  were  determined  by  the 
pitch  of  the  exciting  tone,  their  number  being  doubled  when 
the  pitch  was  raised  by  an  octave  and  halved  when  the 
pitch  was  lowered  by  an  octave.  Each  tone  was  found  to 
produce  its  peculiar  sound  picture ;  simultaneously  occurring 
tones  formed  their  respective  sound  pictures  independently  ; 
noises  produced  a  series  of  continuously  changing  sound 
pictures.  Believing  that  similar  sound  pictures  might  be 
formed  on  the  minute  basilar  membrane  of  the  cochlea, 
Ewald  later  endeavoured  to  construct  an  artificial  mem- 
brane more  closely  resembling  it  in  size  and  delicacy. 
From  an  aluminium  disc  he  cut  out  a  rectangular  slit  about 
eight  millimeters  in  length  and  half  a  millimeter  in  breadth. 
He  plunged  the  disc  in  a  solution  of  rubber  in  benzine, 
which  on  evaporation  left  a  minute  and  extremely  thin 
rubber  membrane  over  the  slit.  The  disc  was  then  sur- 
rounded by  fluid  in  a  chamber,  one  wall  of  which  was  fitted 
with  an  artificial  fenestra  ovalis  connected  by  a  solid  rod 
with  an  outer  membrane  representing  the  tympanic  mem- 
brane. A  beam  of  light  was  thrown  on  to  the  artificial 
basilar  membrane,  the  formation  of  sound  pictures  on  which 
was  observed  through  an  adjustable  microscope,  or  was 
photographed  by  a  camera  attached  to  the  microscope. 
Ewald  found  that  such  a  delicate  membrane  would  respond 
by  giving  sound  pictures  through  a  range  of  more  than  six 
octaves,  when  the  artificial  tympanic  membrane  was  thrown 
into  vibration  by  tones  reaching  it  from  the  air.  He  called 
his  chamber  a  "camera  acustica,"  urging  that  the  sound 
pictures  produced  in  it  veritably  represent  the  behaviour 


AUDITORY  SENSATIONS  61 

of  that  part  of  the  cochlea  basilar  membrane  which  under- 
lies the  tunnel  of  Corti.  Ewald's  theory,  that  each  tone 
produces  its  own  sound  picture  on  the  basilar  membrane, 
and  that  the  tone  sensation  depends  on  the  particular  hair 
cells  which  are  stimulated  by  the  waves  or  lines  of  the 
sound  picture,  was  at  first  considered  to  be  even  less 
probable  than  that  of  Helmholtz,  until  he  recently  demon- 
strated the  phenomena  in  this  way  on  an  artificial  model. 
Such  a  demonstration  could  not  fail  to  advance  his  theory 
in  general  favour.  Much  more,  however,  remains  to  be 
done  before  it  can  be  fairly  weighed  with  the  older  reson- 
ance theory.  We  must  know  whether  such  a  membrane 
can  accurately  represent  the  more  complex  structure  of 
the  basilar  membrane,  whether  it  can  completely  picture 
difference  tones,  how  it  responds  to  beats  and  to  changes  of 
phase,  and  how  the  analysis  of  simultaneously  sounding 
tones  into  their  components  is  conceivable.  Meanwhile  we 
can  only  hold  a  suspended  judgment. 

Meyer  s  Theory. — Another  theory  of  hearing,  dispensing 
with  the  necessity  of  analysis  by  resonators  in  the  cochlea, 
may  be  briefly  noticed.  Max  Meyer  suggested  that  the 
loudness  of  a  tone  is  determined  by  the  extent  of  basilar 
membrane  bent  out  according  as  the  thrust  of  the  stapes  is 
strong  or  feeble.  He  suggested  that  the  pitch  of  a  tone  is 
determined  by  the  frequency  with  which  the  basilar  mem- 
brane is  so  bent,  the  frequency  being  dependent  on  the 
number  of  thrusts  per  second  of  the  stapes.  According  to 
this  view,  the  individual  movements  of  the  stapes  determine 
the  sensation  produced  in  the  cochlea,  a  loud  sensation 
arising  when  many  hair  cells  are  thrown  into  vibration  by 
a  single  thrust  of  the  stapes,  a  sensation  of  high  pitch 
arising  when  any  given  hair  cell  is  stimulated  with  sufficient 
frequency  by  rapid  movements  of  the  stapes.  The  theory, 
however,  fails  to  account  for  difference  tones  satisfactorily, 
and  would  compel  us  to  admit  the  influence  of  phase 
differences  on  auditory  sensations  (page  54). 


62  EXPERIMENTAL  PSYCHOLOGY 


BIBLIOGRAPHY. 

In  addition  to  the  works  mentioned  in  the  last  chapter— C.  Stumpf, 
"  Konsonauz  u.  Dissonanz,"  Bcitr.  zur  Akustik  u.  Musikiviss.,  1898,  i.  1. 
M.  Meyer,  "Zur  Theorie  d.  Differenztone  u.  d.  Gehorsempiindungen 
iiberhaupt,"  Ztscli.  f.  Psychol.  u.  Physiol.  d.  Sinnesorg.,  1898,  xvi.  1  ; 
"An  Introduction  to  the  Mechanics  of  the  Inner  Ear,"  Univ.  of  Missouri 
Studies,  Sci.  Series,  1907,  ii.  No.  1.  K.  Ewald,  "Zur  Physiol.  d.  Laby- 
rinths," Arch.  f.  d.  gc*.  Physiol.,  1899,  Ixxvi.  147  ;  1903,  xciii.  485. 
F.  Kriiger,  "  Zur  Theorie  d.  Combination stone,"  Philosoph.  Stud.,  1901, 
xvii.  185.  K.  L.  Schafer  u.  0.  Abraham,  "Studien  liber  Unterbrechungstone," 
Arch.f.  d.  gcs.  Physiol.,  1901,  Ixxxviii.  207.  0.  Abraham,  "Das  absolute 
Tonbewusstsein,"  Sammelb.  d.  internat.  MusikgeselL,  1901,  iii.  1.  C.  S. 
Myers,  "The  Ethnological  Study  of  Music  ",  among  A  nth  ropological  Essays, 
Oxford,  1908.  L.  Hermann,  "Neue  Untersuchungen  iiber  d.  Natur  d. 
Kombinationstone,"  Arch.f.  d.  gcs.  Physiol.,  1908,  cxxii.  419. 


CHAPTEK  V 
ON   LABYRINTHINE  AND   MOTOR  SENSATIONS 

The  Resemblance  of  these  Sensations.  —  We  may  con- 
veniently consider  these  two  classes  of  sensations  in  the 
same  chapter,  since  they  are  both  closely  related  to 
the  position  and  to  the  movements  of  the  individual. 
The  end  organs,  subserving  the  development  of  laby- 
rinthine sensations,  are  contained  in  the  inner  ear;  the 
nervous  impulses  are  conveyed  by  the  vestibular  division 
of  the  auditory  nerve.  The  end  organs,  that  are  con- 
cerned in  motor  sensations,  are  situated  in  the  motor 
apparatus. 

Motor  sensations  are  often  called  "  kinsesthetic "  sensa- 
tions. But,  strictly  speaking,  the  labyrinthine  sensations 
are  likewise  kinsesthetic  in  function. 

The  sensations,  developed  by  the  motor  apparatus  and 
by  the  labyrinth  of  the  ear,  further  resemble  one  another 
in  their  vague  and  unprojected  character,  and  in  the 
relatively  insignificant  place  they  occupy  in  the  field 
of  consciousness.  Moreover,  the  end  organs  of  the  motor 
apparatus  and  of  the  labyrinth  are  alike  engaged  in  con- 
tinuously transmitting  impulses  by  which  muscular  tone 
is  reflexly  maintained  and  movements  are  unconsciously 
co-ordinated. 

A  study  of  these  sensations  impresses  on  us  the  important 
fact  that  psychology  must  needs  take  into  account  not 
merely  the  conscious  but  also  the  unconscious  aspects  of 

psycho-physiological  processes. 

63 


64  EXPERIMENTAL  PSYCHOLOGY 

LABYRINTHINE  SENSATIONS. 

The  End  Organs, — The  end  organs  of  the  semicircular 
canals,  perhaps  together  with  those  of  the  utricle  and  saccule, 
are  the  seat  of  sensations  by  the  aid  of  which  we  become 
aware  of  the  position,  and  of  movements,  of  the  head.  They 
also  transmit  nervous  impulses,  which  reflexly  co-ordinate 
the  movements  of  the  eyes,  head,  and  body ;  and  they  aid 
in  maintaining  the  tone  and  reflex  activity  of  the  general 
muscular  system. 

Experimental  Interference  with  the  Canals. — The  effect 
of  experimentally  stimulating  a  single  canal  is  to  pro- 
duce movements  of  the  head  in  the  plane  of  the  canal 
stimulated.  The  effect  of  destroying  a  canal  depends  on 
the  amount  of  stimulation  produced  during  the  operation. 
With  special  precautions  a  canal  may  be  cut  through  so  as 
to  yield  practically  no  ill  effects.  If,  however,  the  corre- 
sponding canal  on  the  opposite  side  be  then  removed,  un- 
controllable pendular  movements  of  the  head  are  set  up, 
together  with  other  disturbances  in  eye  movement  and  in 
general  co-ordination,  which  we  shall  consider  immediately. 

After  removal  of  the  labyrinth  on  one  side,  the  resulting 
disturbance  is  in  great  part  asymmetrical;  it  is  also  less 
obvious  and  more  transient  than  the  disturbance  following 
the  removal  of  both  labyrinths.  In  the  latter  event,  a  most 
striking  lack  of  co-ordination  is  brought  about.  The  animal 
falls  hither  and  thither,  and  spins  round,  often  irresistibly 
damaging  itself;  its  eyes  are  in  a  state  of  almost  incessant 
movement.  If  a  bird,  it  is  unable  to  fly  properly  or  to  pick 
up  its  food,  save  with  great  difficulty. 

In  course  of  time,  these  uncontrollable  movements  of  the 
head,  eyes,  and  body  begin  to  disappear ;  but  for  some  days 
or  weeks,  there  may  be  a  loss  of  co-ordination  and  attacks  of 
apparent  giddiness,  whenever  movements  of  the  head  are 
attempted.  So  long  as  the  head  is  at  rest,  the  results  of 
the  operation  are  almost  negligible.  Sooner  or  later,  this 


LABYRINTHINE  AND  MOTOR  SENSATIONS     65 

inco-ordination  and  giddiness,  together  with  other  disturb- 
ances (which  need  not  concern  us  here)  likewise  vanish  ; 
the  most  lasting  effects  of  destruction  of  the  canals  con- 
sisting in  a  general  loss  of  muscular  tone  and  power,  and  in 
a  diminution  of  reflexes. 

The  Mach-Breuer-Brown  Theory. — It  has  been  suggested 
that  the  end  organs  of  the  canals  are  stimulated  by  changes 
in  pressure,  due  to  movements  in  the  fluid  which  surrounds 
the  end  organs.  Upon  this  hypothesis,  the  start  of  any 
head  movement  must  initiate  a  stimulus,  owing  to  the 
tendency  of  the  fluid  to  lag  behind  and  so  to  press  on  the 
end  organs  of  one  or  more  canals,  according  to  the  plane 
of  the  movement.  But  as  soon  as  the  fluid  and  the  end 
organs  move  with  equal  speed,  that  stimulus  must  cease. 
A  fresh  stimulus  will  occur  when  the  movement  of  the 
head  ceases,  the  fluid  tending  by  its  own  inertia  to  continue 
in  motion  and  thus  to  stimulate  the  now  resting  canals.  In 
consideration  of  the  microscopic  calibre  of  the  semicircular 
canals,  this  hypothesis  has  since  been  slightly  modified,  in 
detail  only,  but  not  in  principle.  It  was  urged  that  no 
appreciable  movement  of  fluid  could  occur  within  canals 
the  diameter  of  which  measured  in  man  one-tenth  of  a 
millimeter,  and  in  birds  much  less.  Instead  of  the  displace- 
ments of  fluid  en  masse,  the  wave-like  action  of  thrusts  of 
fluid  on  the  end  organs  was  accordingly  substituted. 

This  hypothesis,  brought  forward  by  Mach,  Breuer,  and 
(in  its  original  form  only)  by  Crum  Brown,  implies  that  the 
canals  are  stimulated  only  by  change  from  a  state  of  rest  to 
one  of  movement  of  the  head  (or  vice  versd),  or  by  change  of 
rate  of  movement  of  the  head.  Experiment  shows  that  this 
is  actually  the  case.  When  an  individual  is  passively 
rotated  with  closed  eyes,  he  soon  loses  all  sensation  of 
movement,  so  long  as  the  plane  and  the  rate  of  rotation  are 
constant,  and  so  long  as  he  does  not  move  his  head.  But 
he  at  once  appreciates  any  actual  acceleration  of  movement ; 
and  if  he  move  his  head,  the  sensation  of  rotation,  previously 
5 


66  EXPERIMENTAL  PSYCHOLOGY 

lost,  returns.  Moreover,  when  the  speed  is  retarded  or 
when  rotation  is  actually  stopped,  he  is  under  the  illusion 
that  the  direction  of  rotation  is  reversed.  It  is  further 
found  that,  after  various  changes  in  the  direction  of  rotation 
have  rapidly  succeeded  one  another,  his  judgments  of  the 
direction  are  apt  to  be  erroneous. 

Eye  Movements  produced  by  Rotation. — When  the  eyes 
are  open  during  active  or  passive  rotation  of  the  body,  it 
will  be  observed  that  at  the  start  of  rotation  they  fix  a 
stationary  object  and  then  jerk  forwards  in  the  direction  of 
rotation,  that  they  then  fix  another  object  and  again  jerk 
forwards,  and  so  on.  These  forward  movements  are  so  rapid 
that  no  account  is  taken  of  the  visual  sensations  of  move- 
ment arising  therefrom.  Hence  to  the  individual,  at  this 
stage,  external  objects  appear  stationary.  As  rotation  con- 
tinues, the  eyes  no  longer  rest  but  passively  follow  the 
movements  of  the  body,  so  that  external  objects  now  seem 
to  be  moving  in  a  direction  opposite  to  that  of  rotation. 
After  rotation  of  the  body  has  ceased,  the  eyes  continue  for 
some  time  to  move  involuntarily  in  the  same  direction 
as  before,  intermittently  swinging  suddenly  back,  again 
passively  moving  on  and  then  swinging  back.  Objects  thus 
appear  to  continue  moving  in  the  same  direction  as  before. 
After  rotation  of  the  body  has  ceased,  there  is  a  strong  and 
often  irresistible  tendency  to  recommence  rotation. 

Head  Movement  in  Rotation. — The  axis,  about  which 
external  objects  appear  to  rotate  when  the  body  has  come 
to  rest,  depends  on  the  position  of  the  head  in  relation  to 
the  axis  of  rotation.  If  during  or  immediately  after  rotation 
the  head  be  moved  (e.g.  if  it  be  inclined  to  one  shoulder  or 
if  the  face  be  turned  to  the  ceiling  or  to  the  ground),  the 
body  appears  to  be  rotating  in  a  different  direction  and  the 
axis  of  rotation  of  external  objects  is  changed. 

Giddiness. — Giddiness  is  experienced  when  a  galvanic 
current  is  transmitted  through  the  ears.  An  illusion  of 
falling  towards  the  cathode  pole  arises,  to  counteract  which 


LABYRINTHINE  AND  MOTOR  SENSATIONS     67 

the  subject  may  actually  fall  towards  the  anode  pole. 
Giddiness  is  a  common  symptom  of  Meniere's  disease,  in 
which  the  functions  of  the  semicircular  canals  are  un- 
questionably disturbed.  The  giddiness  which  ensues  on 
rotation  of  the  body  is  absent  or  deficient  in  a  certain 
proportion  of  deaf  mutes ;  this  proportion  being  so  similar 
to  the  frequency  of  defective  canals  found  in  deaf  mutes 
after  death,  as  to  suggest  a  definite  relation  between 
giddiness  and  the  proper  function  of  the  semicircular 
canals.  In  somewhat  like  proportion,  deaf  mutes  fail  to 
show  the  usual  eye  movements  which  occur  during  rotation 
of  the  body.  They  also  find  difficulty  in  walking  in  a 
straight  line,  in  standing  on  one  leg,  or  in  otherwise 
balancing  themselves,  when  their  eyes  are  shut.  There  is 
thus  ground  for  believing  that  the  movements  which  occur 
during  rotation,  and  that  the  co-ordinations  which  occur 
during  rest  of  the  head  (cf.  page  64),  are  the  reflex  results 
of  stimulation  of  the  semicircular  canals ;  and  that  giddi- 
ness arises  when  there  is  a  discrepancy  or  confusion  between 
the  various  labyrinthine,  retinal,  cutaneous,  and  motor 
sensations  which  inform  us  of  the  position  of  the  body 
relatively  to  the  external  world. 

The  giddiness  which  so  commonly  arises  from  sudden, 
and  for  the  moment  inexplicable,  alterations  of  our  sur- 
roundings (for  example,  when  we  are  gazing  at  a  mirror 
which  is  suddenly  blown  by  the  wind),  while  our  feet  and 
body  are  stationary,  may  be  ascribed  to  a  similar  discrepancy 
of  sensations  and  to  a  similar  confusion.  Giddiness  is  also 
apt  to  occur  in  any  unfamiliar  strain  upon  the  eyes,  for 
example,  when  strange  glasses  are  worn  and  unusual 
binocular  movements  are  required,  or  when  such  glasses 
are  removed  after  adaptation  thereto.  It  may  occur  as 
the  result  of  very  rapid  stimulation  or  movements  of  the 
eyes,  e.g.  in  regarding  flicker,  or  in  watching  a  waterfall.  It 
occurs  in  disturbances  of  circulation  and  in  disease  of  the 
central  nervous  system,  and  is  induced  by  certain  drugs,  e.g. 


68  EXPERIMENTAL  PSYCHOLOGY 

alcohol  and  tobacco.  In  each  case  there  is,  for  various 
reasons,  a  disturbance  in  co-ordination,  manifesting  itself, 
objectively,  in  disordered  movements  (which  in  part,  at 
least,  seek  to  repair  the  disturbance)  and,  subjectively,  in 
the  experience  of  giddiness. 

The  Utricle  and  Saccule. — The  functions  of  the  maculae 
of  the  utricle  and  saccule  are  quite  uncertain.  It  is  said 
that  the  positions  of  the  two  maculae  are  exactly  per- 
pendicular to  one  another,  and  that  they  favour  horizontal 
and  prevent  vertical  movement  of  the  otoliths  which 
they  contain.  And  it  has  been  suggested  that  their  hair 
cells  may  be  stimulated  by  the  overlying  plate  of  otoliths, 
giving  rise  to  sensations  which  are  interpreted  either  as 
change  in  inclination,  or  as  translatory  motion,  of  the  head, 
according  as  the  movement  involves  simultaneous  excitation 
of  the  semicircular  canals  or  not.  But  there  are  many 
difficulties  in  the  way  of  accepting  this  hypothesis. 

MOTOR  SENSATIONS. 

In  addition  to  having  a  labyrinthine  origin,  sensations  of 
movement  also  originate  in  the  locomotor  apparatus  of  the 
body.  We  are  aware  when  in  the  dark  we  have  actively 
moved  our  finger,  or  when  it  has  been  moved  for  us 
passively;  we  know  roughly  the  speed,  the  direction,  and 
the  extent  of  this  movement  (exps.  39,  42). 

Their  Non-cutaneous  Origin. — It  is  easily  demonstrable 
that  such  kinaesthetic  experiences  are  quite  independent  of 
cutaneous  sensations.  In  cases  of  locomotor  ataxia,  the 
former  are  to  a  great  extent  abolished  while  the  sensibility 
of  the  skin  may  be  unimpaired.  With  their  eyes  shut,  such 
patients  may  be  unaware  whether  or  to  what  extent  their 
limbs  have  been  actively  or  passively  moved ;  they  are 
unable  accurately  to  co-ordinate  their  movements,  or  to 
estimate  the  position  of  or  the  resistance  offered  to  their 
limbs.  In  normal  individuals,  the  skin  may  be  anaesthetised 


LABYRINTHINE  AND  MOTOR  SENSATIONS    69 

by  the  application  of  cocaine  or  by  a  faradic  current,  and 
yet  the  kinaesthetic  sensations  are  as  acute  as,  if  indeed  not 
more  acute  than  before  (exps.  40,  41). 

The  End  Organs. — These  sensations  of  movement  con- 
ceivably originate  in  muscles,  tendons,  or  joints.  They  are 
indeed  often  called  "muscular"  sensations,  from  a  long- 
standing belief  that  they  are  produced  by  excitation  of  the 
sensory  structures  (e.g.  muscle  spindles)  which  are  con- 
tained and  excited  within  the  contracting  or  relaxing 
muscles.  Various  reasons,  however,  have  been  advanced  in 
favour  of  the  view  that  these  sensations  are  chiefly  of 
articular  origin.  In  the  first  place,  some  muscles,  e.g.  the 
long  flexors  and  extensors  of  the  digits,  lie  at  a  consider- 
able distance  from  the  joints  on  which  they  act  and  at 
which  we  localise  the  resulting  movements ;  these  muscles 
being  connected  to  the  bones  by  very  long  tendons. 
Secondly,  it  has  been  found  that  the  smallest  perceptible 
movement  is  approximately  the  same  for  passive  as  for 
active  movement.  Both  arguments  obviously  lack  cogency  : 
the  former  because  it  leaves  out  of  account  the  influence  of 
the  tendons,  the  latter  because  it  overlooks  the  changes 
in  form  or  tension  of  these,  and  their  antagonistic, 
muscles  during  passive  extension  or  flexion.  Such  changes 
might  lead  to  excitation  of  the  contained  sensory  organs. 

Of  greater  weight  is  the  observation  that  a  faradic 
current,  when  passed  through  a  joint,  raises  the  threshold 
for  active  or  passive  movement  considerably.  Thus,  whereas 
under  ordinary  conditions  a  movement  of  the  first  inter- 
phalangeal  joint  can  be  just  perceived  when  it  is  passively 
moved  through  an  angle  of  0°'5,  it  has  been  found  that  the 
threshold  is  raised  to  1°*5,  2°'5  or  even  to  30<85  (according 
to  the  strength  of  the  current),  when  a  current  is  passed 
through  the  joint  during  movement.  The  movements  of 
the  finger  produced  under  these  conditions,  especially  when 
the  eyes  are  closed,  are  described  as  jerky  and  inco- 
ordinated,  like  those  of  an  ataxic  patient  (exp.  44). 


70  EXPERIMENTAL  PSYCHOLOGY 

In  support  of  the  articular  source  of  kinsesthetic 
sensations,  attention  has  also  been  called  to  the  imperfect 
sense  of  movements  and  of  position  possessed  by  jointless 
organs,  such  as  the  lips,  the  tongue,  and  the  eyeballs, 
when  the  contributing  aid  of  tactile  or  visual  sensations 
is  excluded  (page  278).  Moreover,  in  certain  cases  of  loco- 
motor  ataxia,  the  kinsesthetic  sense  has  been  found  to 
become  still  blunter  by  pulling  the  joint  surfaces  apart 
from  one  another.  It  seems  reasonable,  then,  to  conclude 
that  articular  sensations  are  a  very  important  determinant 
of  our  awareness  of  movement. 

Characters  of  Kincesthesis. — In  our  everyday  movements, 
the  attention  is  primarily  concerned  with  the  aim  of  the 
intended  movement,  or  with  the  general  situation  of  which 
it  forms  part;  we  are  but  dimly  conscious,  or  often  quite 
unconscious,  of  the  kinsesthesis  itself.  Such  sensations 
of  movement  as  we  have  are  supplemented  and  greatly 
obscured  by  visual  experiences.  It  is  only  when  new 
movements  are  being  carried  out,  or,  more  especially, 
when  prescribed  movements  are  made  with  the  eyes  closed, 
that  the  true  nature  and  importance  of  kinsesthesis  are 
appreciable. 

Under  such  conditions  it  is  easy  to  demonstrate  that 
there  are  three  directions  in  which  sensations  of  movement 
may  vary,  namely,  in  extent,  duration,  and  quality.  The 
kinsesthesis  in  a  short  movement  is  obviously  less  extensive 
than  that  in  a  longer  movement.  The  kinsesthesis  in  a 
slowly  moving  limb  lasts  longer  than  that  in  a  more  rapidly 
moving  limb.  The  kinaesthesis  in  the  toe  or  elbow  is  of 
different  quality  from  that  in  the  finger  or  shoulder.  It  is 
difficult  to  see  how  sensations  of  mere  movement  can  be 
more  or  less  intense ;  but  other  forms  of  motor  sensation 
(cf.  page  71)  are  clearly  characterised  by  intensity. 

Illusions  of  Extent  of  Movement.  —  The  duration  of 
kinsesthesis  is  one  of  the  factors  underlying  our  estimation 
of  the  range  or  extent  of  movement.  A  slow  movement 


LABYRINTHINE  AND  MOTOR  SENSATIONS     71 

appears  longer  than  a  rapid  movement  of  the  same  range. 
Our  estimation  of  the  range  of  movement  is  also  dependent 
on  the  relation  between  the  duration  of  movement  and  the 
degree  of  muscular  effort  put  forth. 

Other  Motor  Sensations. — When  we  come  to  consider  our 
appreciation  of  weight,  we  shall  find  reason  to  believe  that 
our  awareness  of  muscular  effort  depends  on  two  other 
classes  of  motor  sensations, — sensations  of  deep  pressure 
and  sensations  of  strain  or  tension,  which  are  principally  of 
muscular  and  tendinous  origin  (exp.  43).  We  may  note, 
in  passing,  that  sensations  of  movement,  deep  pressure,  and 
tension  by  no  means  exhaust  the  list  of  sensations  yielded 
by  the  motor  apparatus.  We  have,  for  example,  sensations 
of  cramp,  residing  in  the  muscles,  sensations  of  fatigue, 
doubtless  common  to  muscles,  tendons,  and  joints,  and  the 
articular  sensations  of  friction  and  of  contact,  which  occur 
during  active  or  passive  rotation  of  joints  and  during  sudden 
shocks  or  jars. 

Other  Factors  determining  Extent  of  Movement. — When, 
in  the  blindfold  subject,  the  arms  start  from  a  symmetrical 
position  (the  mid-line  of  the  body),  and  simultaneously 
execute  intentionally  equal  and  similar  movements  (e.g.  the 
right  arm  moving  to  the  right,  the  left  arm  to  the  left), 
these  movements  are  usually  unequal.  One  of  the  subject's 
arms  habitually  travels  the  farther :  that  is  to  say,  he 
underestimates  the  extent  of  its  movement.  Probably,  as  a 
rule,  the  preferred  arm  executes  the  longer  movement, — 
the  right  arm  in  right-handed,  the  left  in  left-handed 
individuals.  But  this  is  not  invariably  the  case ;  for  the 
subject  may  make  allowance  for  the  readier  movement  of 
one  arm,  and,  indeed,  may  make  excessive  allowance.  A 
further  complication  may  arise  from  the  fact  that,  of  two 
movements,  that  which  requires  or  receives  the  more 
attention,  tends  to  be  overestimated,  the  corresponding 
kinsesthetic  experience  being  the  more  marked. 

When  the   arms   move   successively,  instead  of  simul- 


72  EXPERIMENTAL  PSYCHOLOGY 

taneously,  the  two  movements  are  much  more  nearly  equal. 
In  this  case  the  task  is  easier,  as  the  attention  is  now 
undivided. 

It  is  generally  believed  that  when  two  movements  are 
successively  carried  out  by  the  same  group  of  muscles,  that 
movement  tends  to  be  the  greater  which  is  effected  by  the 
initially  less  contracted  muscle.  For  example,  if  the  arm 
make  two  successive,  intentionally  equal,  downward  move- 
ments, that  movement  is  the  longer  which  is  begun  in  the 
more  downward  position  of  the  arm.  This  explanation, 
however,  is  for  several  reasons  unsatisfactory.  For,  if  a 
whole  series  of  successive  and  apparently  equal  movements 
are  executed  by  the  arm  (moving  from  right  to  left  or  from 
above  downwards),  it  appears  that  both  the  initial  and  the 
terminal  movements  are  overestimated  relatively  to  the 
intermediate  movements. 

We  have  to  bear  in  mind  that  our  appreciation  of  the 
muscular  effort  put  forth,  upon  which,  as  we  have  said,  our 
estimate  of  movement  in  part  depends,  is  invariably  the 
result  of  activity,  not  in  a  single  muscle,  but  in  a  group  of 
muscles.  The  degree  to  which  different  muscles  are  involved 
in  the  movement  of  a  limb  varies  widely  in  different  stages 
of  its  excursion.  Towards  the  completion  of  a  prescribed 
movement  (as  in  lifting  the  arm  above  the  head),  fresh 
muscles  may  be  brought  into  action,  which  were  not  engaged 
at  the  start  of  that  movement. 

We  have,  therefore,  to  take  into  account  the  total 
experience  of  muscular  effort  put  forth ;  we  cannot  state  the 
error  of  estimation  in  terms  of  the  degree  of  contraction  of 
a  single  muscle.  The  overestimation,  met  with  in  the 
initial  and  final  portions  of  a  subdivided  arm  movement 
probably  finds  its  readiest  explanation  in  the  relatively 
greater  ease  and  freedom  with  which  the  intermediate  series 
of  movements  is  made.  In  the  extreme  positions  of  an  arm, 
a  greater  force  of  muscular  contraction  is  necessary;  and 
this  greater  exertion  produces  the  illusion  of  more  extensive 


LABYRINTHINE  AND  MOTOR  SENSATIONS     73 

movement.  Similarly,  inasmuch  as  the  leg  habitually 
executes  freer  and  more  extensive  movements  than  the  arm, 
the  leg  actually  moves  through  a  wider  range  than  the  arm, 
while  to  the  blindfold  subject  they  appear  to  execute  equal 
movements  (exp.  44). 

Awareness  of  Position. — It  might  be  thought  that  our 
appreciation  of  movement  is  dependent  on  our  appreciation 
of  change  of  position.  But  we  have  indications  that  such  a 
comparison  of  positions  does  not  necessarily  take  place. 
The  effect  of  transmitting  an  electric  current  through  a 
joint  is  to  obliterate  awareness  of  position,  while  awareness 
of  movement  (although  much  more  obtuse  than  in  the 
absence  of  faradisation)  is  still  preserved.  The  movement 
of  a  finger  may  be  recognised  when  it  is  of  so  slight  a 
duration  (an  extremely  small  fraction  of  a  second)  that  it  is 
difficult  to  suppose  that  any  discrimination  between  a  series 
of  positions  has  taken  place.  In  the  third  place,  a  passive 
movement  may  be  recognised,  and  yet  the  direction  of 
movement  and  hence  the  nature  of  the  change  of  position 
may  be  doubtful  (exp.  42). 

There  is,  indeed,  more  reason  to  think  that  our  awareness 
of  position  is  dependent  on  that  of  movement  than  that  the 
reverse  relation  holds.  When  our  limbs  are  screened  from 
the  eyes,  and  are  kept  unmoved  for  a  considerable  time,  or 
if  they  be  passively  moved  while  our  attention  is  distracted, 
we  find  great  difficulty  in  determining  their  position  without 
moving  them.1  In  awakening  from  sleep,  we  are  confronted 
with  a  like  difficulty.  But  while  movement  greatly  aids 
our  judgment  of  position,  the  latter  is  unquestionably  in 
part  dependent  on  what  we  may  term  "  stataesthetic  "  sensa- 
tions, that  is  on  sensations  which,  having  presumably  the 
same  origin  as  kinresthetic  sensations,  are  developed  during 
rest  of  the  mobile  organs.  Our  sense  of  position  is  further 
clearly  dependent  on  that  complex  series  of  earlier  visual, 

1  Recent  experiments,  however,  throw  doubt  upon  the  general  truth  of 
this  statement. 


74  EXPERIMENTAL  PSYCHOLOGY 

tactile,  and  kinsesthetic  experiences,  the  sources  of  our 
conception  of  form  and  space. 

Comparison  between  the  Nervous  Connections  of  the  Motor 
and  Labyrinthine  Sensory  Apparatus. — It  may  be  worth 
while,  before  we  close  this  chapter,  briefly  to  compare 
the  connections  which  the  afferent  motor  and  vestibular 
nerves  respectively  make  with  the  central  nervous  system. 

A  voluntary  (skeletal)  muscle  is  controlled,  in  the  first 
place,  by  a  lower  system  of  "  nuclear  "  nervous  arcs,  the 
centres  of  which  lie  in  the  sensory  cells,  and  in  the  cells  of 
the  motor  nuclei,  of  the  spinal  cord,  bulb,  and  mid-brain. 
The  efferent  parts  of  these  nuclear  arcs  consist  of  the  cells 
of  the  motor  nuclei,  together  with  their  axis  cylinders, 
which  terminate  in  the  end  plates  of  the  striated  muscular 
fibres.  Their  afferent  parts  are  derived  from  peripheral 
sensory  nerve  fibres  which  run  from  various  muscular, 
tendinous,  cutaneous,  and  other  tissues  towards  the  sensory 
cells,  to  terminate  ultimately  around  the  cells  of  the  motor 
nuclei. 

This  system  of  nuclear  arcs  is,  in  turn,  controlled  by 
higher  series  of  arcs,  situated  in  the  cerebral  hemispheres,  in 
the  cerebellum,  and  possibly  elsewhere.  The  centres  of  the 
cerebral  "  cortical "  arcs  lie  in  the  cells  of  the  sensory  and 
motor  cortex.  Their  efferent  parts  consist  of  these  cortical 
motor  cells,  and  of  their  axis  cylinders  which  descend  in 
certain  columns  of  the  mid-brain,  bulb,  and  cord,  terminating 
around  the  motor  nuclei.  Their  afferent  parts  come  from 
muscular,  tendinous,  cutaneous,  and  other  structures,  ascend- 
ing within  the  cord,  and  (with  the  aid  of  relays)  passing 
to  the  cerebral  cortex.  Probably  the  cerebellar  arcs  have 
a  similar  constitution. 

Finally,  higher  systems  of  arcs  exist,  overlying  and 
co-ordinating  the  above  systems. 

The  vestibular  nerve  resembles  in  its  cerebellar  con- 
nections those  of  the  afferent  nerves  of  the  motor  apparatus  : 
its  relation  to  the  cerebral  cortex  is  little  known.  The 


LABYRINTHINE  AND  MOTOR  SENSATIONS    75 

cerebellum  is  the  great  centre  where  afferent  impulses,  alike 
from  the  labyrinthine  and  motor  apparatus,  are  gathered 
together.  From  the  cerebellum  efferent  impulses  proceed 
to  the  fore-brain  and  to  the  mid-brain,  bulb,  and  cord, 
influencing  the  discharge  of  impulses  along  the  efferent 
neurons  of  the  nuclear  arcs. 


BIBLIOGRAPHY. 

A.  Crnm  Brown,  "On  the  Sense  of  Rotation,"  etc.,  J.  of  Anat.  and 
Physiol.,  1874,  viii.  327.  E.  Much,  Grundlinien d.  Lehre  v.  d.  Bewegungsemp- 
ftndungen,  Leipzig,  1875  ;  Contributions  to  the  Analysis  of  the  Sensations, 
Eng.  trans.,  Chicago,  1897.  J.  Breuer,  "  Ueber  d.  Function  d.  Otolithen- 
apparate,"  Arch.  /.  d.  ges.  Physiol. ,  1890,  xlviii.  195;  (with  similar  title), 
ibid.,  1897,  Ixviii.  596  ;  Sitzungsber.  Wiener  Akad.  d.  Wissensch.,  math.-nat. 
CL,  1903,  cxii.  Abt.  3,  315.  A.  Goldscheider,  GesammeUe  Abhandlungen, 
Leipzig,  1898,  ii.  V.  Henri,  "Revue ge'ne'rale  sur  le  sensmusculaire,"J7^?me'e 
psychologigue,  1899,  5me  Annee,  399.  R.  S.  Wood  worth,  Le  Afouvement, 
Paris,  1903.  W.  Nagel,  "  Die  Lage-,  Bewegungs-  und  Widerstandsemp- 
findungen,"  inNagel'sffandbuchd.  Physiol.  d.  Menschen,  Braunschweig,  1905, 
iii.  735.  W.  Peters,  "  Die  Bewegungs-  u.  La.geempfindungen,"  Arch.  /.  d. 
ges.  Psychol.  (Literaturbeiicht),  1905,  v.  42.  E.  Jaensch,  "Ueber  d. 
Beziehungen  von  Zeitschatzung u.  Bewegungsempfindung,"  Ztsch.  f .Psychol., 
1906,  xli.  257.  C.  Spearman,  "Die  Normaltauschungen  in  d.  Lagewahr- 
nehmung,"  Psychol.  Stud.,  1906,  i.,  388. 


CHAPTEE    VI 
ON  VISUAL  SENSATIONS1 

The  Characters  of  Visual  Sensations. — Our  visual  sensa- 
tions comprise  colourless  and  colour  sensations.  The  series 
of  colourless  sensations  include  every  shade  of  grey  between 
the  most  blinding  white  and  the  deepest  black  (exp.  45). 
Colour  sensations  include  not  only  the  various  "  spectral " 
hues  which  are  afforded  by  the  analysis  of  daylight,  but 
also  others  which  are  not  to  be  thus  obtained,  e.g.  purple 
and  carmine.  They  differ  in  hue  (or  colour),  intensity, 
saturation,  and  brightness. 

The  normal  (or  "  adequate  ")  stimulus  to  the  retina  is  a 
series  of  wave  movements  in  the  surrounding  ether.  The 
various  hues,  seen  in  the  spectrum,  are  dependent  on  the 
different  lengths  of  these  waves,  sensations  of  red  being 
excited  by  the  longest,  those  of  the  violet  by  the  shortest 
ethereal  waves.  But,  as  we  shall  see  presently,  these 
(and  other)  hues  are  also  obtainable  by  appropriate  mixtures 
of  ethereal  waves. 

The  intensity  of  a  colour  (or  colourless)  sensation  is 
dependent  on  the  intensity  of  the  stimulus,  i.e.  on  the 
amplitude  of  the  light  waves  which  fall  upon  the  retina. 

The  saturation  of  a  colour  sensation  is  dependent  on  the 
amount  of  white  light  that  simultaneously  excites  the  same 
retinal  area  (exp.  46).  The  spectrum  affords,  with  less 
difficulty,  more  highly  saturated  sensations  than  any  other 
external  source  of  stimuli. 

1  See  footnote  to  Chapter  III. 

76 


VISUAL  SENSATIONS  77 

When  highly  saturated  colour  sensations,  e.g.  those 
yielded  by  the  spectrum,  are  compared,  they  obviously 
differ  among  one  another  in  brightness.  Under  ordinary 
conditions  of  vision,  yellow  is  the  region  of  maximal 
"brightness  value."  Brightness  is  a  psychical  character 
which  is  distinct  from  intensity  and  saturation.  Unlike 
these,  it  has  no  obvious  physical  correlate  (exp.  47). 

We  shall  soon  be  in  a  position  to  realise  that  these  four 
characters  of  colour  sensation,  hue,  intensity,  saturation,  and 
brightness,  are  determined  not  merely  by  the  nature  of  the 
stimulus,  but  also  by  the  condition  of  the  cerebro-retinal 
apparatus  at  the  time  of  stimulation. 

[Our  colour  sensations  in  everyday  life  are  greatly  in- 
fluenced by  previous  knowledge.  We  come  to  ascribe  an 
absolute  colour  to  all  familiar  objects,  and  we  either  wholly 
neglect  occasional  variations  in  their  colour,  or  else  treat 
them  as  chance  modifications  of  an  underlying  absolute 
colour.  We  unconsciously  make  allowance  for,  and  hence 
fail  to  observe,  such  differences  in  shade  as  experience  has 
taught  us  to  expect  under  different  conditions  of  illumination 
(exp.  48).  When,  on  the  other  hand,  we  are  forced  to  notice 
the  altered  colour  of  a  well-known  object, — when,  for  ex- 
ample, we  behold  the  rosy  glow  shed  by  the  setting  sun  on  a 
snow  mountain, — we  cannot  resist  the  interpretation  that  we 
are  looking  at  a  really  white  surface  accidentally  reddened 
by  the  peculiar  illumination  under  which  it  is  viewed ;  for 
this  reason  we  tend  to  underestimate  the  degree  of  redness 
of  the  snow.  Nor  are  such  innate  tendencies  dispelled  by 
the  fullest  conviction  that  the  hue  is  determined  both  by 
the  nature  of  the  object  and  by  the  nature  of  the  illumina- 
tion under  which  the  object  is  viewed.] 

The  Conditions  of  Colourless  Sensations. — A  colourless 
sensation  may  be  produced  by  re-combining  all  the  spectral 
colours  obtained  by  the  analysis  of  white  light.  It  may 
also  be  produced  by  combining  merely  three  colours  in 
appropriate  proportions,  provided  that  they  are  properly 


78  EXPERIMENTAL  PSYCHOLOGY 

chosen.  Three  such  colours  are  called  the  "  primary " 
colours.  They  may  be  represented  as  occupying  the  angles 
of  a  triangle  (fig.  1,  page  82),  along  two  of  the  sides  of 
which  may  be  marked  off  spectral  colours  that  are  inter- 
mediate in  wave  length  between  those  at  the  angles.  It  is 
possible  to  produce  any  colour  sensation,  by  mixing  these 
three  colours  (if  necessary,  with  black  and  white)  in  other 
proportions  (exp.  49). 

Within  this  triangle  a  point  W  may  be  found,  which 
will  yield  a  white  sensation  when  a  straight  line  is  drawn 
through  that  point  between  any  points  on  opposite  sides  of 
the  triangle.  That  is  to  say,  every  colour  has  a  correspond- 
ing colour  which,  when  presented  simultaneously  to  the  same 
retinal  area,  produces  a  colourless  sensation.  Such  colours 
are  called  "complementary"  to  one  another  (exps.  50,  51). 

[It  might  be  expected  that  the  hue  of  a  colour  stimulus 
would  become  more  intense,  the  greater  the  intensity  of  the 
stimulus.  But,  beyond  a  certain  limit,  further  increase  of 
the  intensity  of  the  stimulus  causes  the  corresponding 
colour  sensation  to  pass  over  gradually  into  a  colourless 
sensation.  Indeed,  any  colour  stimulus,  if  sufficiently  in- 
tense, is  seen  as  white.  Intense  spectral  reds  and  orange, 
and  greens  up  to  a  wave  length  of  517X,1  acquire  a  yellow 
hue  before  they  in  this  way  become  colourless.  A  green,  of 
a  somewhat  shorter  wave  length  than  517^,  can  be  found, 
which  passes  over  into  white  without  change  of  colour  tone. 
Intense  colour  stimuli,  of  still  shorter  wave  length,  become 
blue  before  they  become  colourless.] 

Under  certain  conditions,  all  colour  stimuli,  if  sufficiently 
feeble,  become  reduced  to  the  colourless  (black-white)  series. 
Conversely,  all  feeble  colour  stimuli  produce,  under  suitable 
conditions,  a  colourless  sensation,  and,  as  they  become 
stronger,  yield  a  colour  (or  " chromatic")  sensation.  The 
conditions  of  this  so-called  "  photochromatic "  interval  will 
be  discussed  later. 

1  X  signifies  a  millionth  part  of  a  millimeter. 


VISUAL  SENSATIONS  79 

The  photochromatic  interval  appears  upon  diminution  of 
a  colour  stimulus  in  extensity,  as  well  as  in  intensity. 
That  is  to  say,  when  the  retinal  area  stimulated  by  the 
colour  is  sufficiently  small,  a  colourless  instead  of  a  colour 
sensation  is  developed. 

All  colour  stimuli  appear  colourless  at  the  extreme 
periphery  of  the  normal  retina.  They  are  seen  as  shades  of 
grey,  the  depth  of  grey  depending  on  the  brightness  of  the 
colour  and  of  the  background  on  which  it  is  seen.  Within 
this  outermost  retinal  zone  of  total  colour  blindness  is  an 
intermediate  zone  of  red-green  blindness,  where  only  blue 
and  yellow  sensations  are  developed.  The  innermost  zone 
is  the  region  of  normal  or  complete  colour  vision.  It  is 
more  correct  to  speak  of  these  as  zones  of  colour  weakness 
rather  than  as  zones  of  colour  blindness,  inasmuch  as  their 
limits  vary  with  the  intensity  and  extent  of  the  colour 
stimulus  (exp.  52). 

Colour  stimuli,  when  acting  for  a  long  period  on  the 
retina,  gradually  fail  to  produce  colour  sensations.  When, 
for  example,  coloured  glasses  are  continuously  worn,  ex- 
ternal objects  are  sooner  or  later  seen  in  their  natural 
colours.  It  can  be  shown  that  the  normal  eye  is  similarly 
"adapted"  to  the  reddish  light  which  enters  the  retina 
through  the  sclerotic  and  iris  (cf.  exp.  132).  Upon  removal 
of  a  colour  stimulus  to  which  the  retina  has  been  adapted,  an 
after-sensation  of  the  complementary  colour  is  developed. 

Successive  Contrast. — If,  after  having  carefully  fixated  a 
coloured  patch,  the  eyes  are  closed  or  are  turned  to  fixate  a 
larger  uniform  surface,  the  form  of  the  coloured  patch 
shortly  appears  as  an  "after-image."  According  to  the 
conditions  of  the  experiment,  this  after-image  will  have  the 
same  brightness  and  the  same  hue  as  the  original  presenta- 
tion, or  it  will  have  the  opposite  degree  of  brightness  and 
the  complementary  hue.  Indeed,  by  projecting  the  after- 
image on  to  surfaces  of  appropriate  hue  or  brightness,  any 
desired  hue  or  brightness  of  the  after-image  may  be  obtained. 


8o  EXPERIMENTAL  PSYCHOLOGY 

The  fixation  of  such  a  coloured  patch  under  ordinary 
conditions  (as  described  in  exps.  53-55)  results  in  an  after- 
effect, or,  more  strictly  speaking,  in  a  series  of  after-effects, 
the  hue  and  brightness  of  which  are  complementary  to 
those  of  the  original  sensation.  These  are  effects  of 
"  successive  contrast."  They  are  termed  "  complementary 
after-sensations,"  or,  more  loosely,  "  negative  images."  We 
shall  defer  our  consideration  of  what  have  been  called 
"  positive "  after-images  and  of  their  relation  to  these 
negative  images  until  later. 

[We  have  spoken  of  a  series  of  after-sensations,  rather 
than  of  a  single  after-sensation.  For  the  after-image 
comes  and  goes,  recurs,  and  again  disappears;  and  this 
happens  several  times  before  the  after-image  vanishes 
altogether.  The  number  and  duration  of  such  fluctuations 
are  said  to  depend  on  the  shape,  size,  and  duration  of  the 
original  stimulus,  and  upon  the  steadiness  of  fixation  while 
the  after-effects  are  being  observed.  Very  large  and  very 
small  after-images  fluctuate  little,  if  at  all.  The  influence  of 
movements  of  the  eyes  upon  the  steadiness  of  after-images 
has  been  the  subject  of  much  controversy.  Some  observers 
believe  that  involuntary  eye  movements  are  an  important,  if 
not  the  principal,  cause  of  the  fluctuation  of  after-images. 
Others  deny  that  such  movements  are  essential,  insisting 
that  periodicity  is  an  inherent  character  of  the  after-image 
process.  In  support  of  the  latter  view,  it  may  be  pointed 
out  that  movement  of  the  background  causes  disappearance 
of  the  after-image  no  less  than  movement  of  the  eyes,  and 
that,  if  two  different  patches  are  successively  fixated  so  as  to 
yield  two  near-lying  after-images  upon  the  retina,  not  only 
are  the  fluctuations  occurring  in  the  one,  not  simultaneous 
with  those  occurring  in  the  other  after-image,  but  this 
want  of  synchronism  is  undisturbed  by  movements  of  the 
eyes. 

The  complementary  effects,  obtained  from  a  given 
retinal  area  after  the  removal  of  its  stimulus,  may  be  also 


VISUAL  SENSATIONS  81 

obtained  during  continued  fixation,  by  diminishing  the 
brightness  or  saturation  of  stimulus.] 

Simultaneous  Contrast. — Phenomena,  analogous  to  those 
of  successive  contrast,  also  occur  owing  to  the  influence 
which  neighbouring  areas  of  the  retina  exert  upon  one 
another ;  they  are  effects  of  "  simultaneous  contrast."  For 
example,  a  given  patch  of  grey  or  of  colour  tends  to  be 
tinged  in  the  colour  complementary  to  that  stimulating  the 
rest  of  the  retina ;  this  is  known  as  "  colour  contrast " 
(exps.  57-60). 

Further,  a  dark  colour  or  a  grey  becomes  brighter  or 
darker,  according  as  it  lies  on  a  background  darker  or 
brighter  than  itself ;  this  is  known  as  "  brightness  contrast  " 
(exps.  61-63). 

Brightness  contrast  is  particularly  well  marked  at  the 
adjoining  margins  of  the  two  contrasting  areas.  It  sub- 
serves the  important  biological  function  of  sharply  outlining 
the  borders  of  seen  objects. 

Colour  contrast  becomes  more  evident,  the  more  nearly 
the  sensations  are  of  equal  brightness,  and  the  more  com- 
pletely any  differences  in  texture  and  the  like  are  obliterated 
between  the  contrasted  surfaces  (exp.  59). 

Both  brightness  contrast  and  colour  contrast  are  in- 
tensified by  increasing  the  extent  or  saturation  of  the 
stimulus  that  evokes  the  contrast  effect. 

[Simultaneous  and  Successive  Induction. — But  these  effects 
of  contrast  disappear  on  prolonged  fixation  and  are  replaced 
by  others  of  a  directly  opposite  character.  The  surface, 
which  at  the  beginning  of  fixation  had  evoked  a  contrast 
colour  or  brightness,  now  induces  its  own  colour  or  bright- 
ness. This  is  termed  "  simultaneous  induction."  Corre- 
sponding changes  in  the  after-image  are  termed  "  successive 
induction  "  (exp.  74).] 

Spectral  Colour  Mixtures. — We  have  seen  (page  78)  that 
three  primary  colours  can  be  chosen,  which  by  admixture  in 
various  proportions  will  give  rise  to  colourless  sensations 
6 


82  EXPERIMENTAL  PSYCHOLOGY 

and  to  colour  sensations  ;  and  not  only  to  the  colour  sensa- 
tions given  by  the  spectrum,  e.g.  orange,  yellow,  and  blue, 
but  also  to  those  not  so  given,  e.g.  purple  and  carmine.  The 
three  colours  which  we  shall  represent  at  the  corners  of  the 
colour  triangle  (fig.  1),  are  red,  green,  and  violet.  Other 
colours  might  have  been  chosen  instead  of  these,  but  one  of 
them  would  necessarily  have  been  a  purple  or  carmine,  and 
would  thus  not  have  obtained  representation  in  the 
spectrum. 

If  two  different  colours,  lying  towards  the  red  end  of  the 
spectrum,  be  thrown  simultaneously  on  the  retina,  the 
resulting  sensation  is  indistinguishable  from  the  sensation 

which  would  have  been 
aroused  by  a  colour  stimulus 
having  a  wave  length  inter- 
mediate between  them.  Thus 
a  given  red  and  a  given 
yellow  stimulus,  when  mixed, 
produce  just  the  same  orange 
p  *  sensation  as  an  orange  spec- 

FIG.  1.  tral  light  would  have   pro- 

duced. The  sensation  result- 
ing from  this  mixture  becomes  more  orange-red  or  more 
orange-yellow,  according  as  the  red  or  the  yellow  stimulus 
preponderates  in  intensity  over  the  other.  Thus  the  effects 
of  mixing  colour  stimuli  at  this  end  of  the  spectrum  may 
be  compared  to  weighting  a  lever  at  its  ends  and  finding  its 
centre  of  gravity. 

Such  conditions,  however,  hold  only  when  that  of  the 
two  colours  which  is  the  more  remote  from  the  red  end,  is 
not  greener  than  a  certain  yellowish-green,  or  more  pre- 
cisely when  its  wave  length  does  not  fall  below  540 >.. 
Beyond  this  point,  the  effect  of  mixing  such  a  colour  with  a 
red  is  to  produce  a  sensation  of  gradually  diminishing 
saturation  ;  until  ultimately  a  green  is  reached  which,  when 
mixed  with  the  red,  produces  a  colourless  sensation.  The 


VISUAL  SENSATIONS  83 

curvature  about  the  apex  of  the  triangle  (fig.  1)  indicates 
the  turning-point,  where  this  diminishing  saturation  begins 
to  occur. 

The  curvature  of  the  line  between  green  and  violet 
similarly  indicates  that  a  mixture  of  green  and  violet 
spectral  lights  yields  a  less  saturated  blue  sensation  than 
would  be  given  by  the  intermediate  blue  spectral  light ;  in 
other  words,  the  resulting  sensation  lies  nearer  the  point 
marked  W.  The  effect  of  mixing  spectral  red  and  violet  is 
to  produce  carmine  or  purple  colour  sensations,  according 
to  the  preponderance  of  the  red  over  the  violet  or  of  the 
violet  over  the  red  stimulus. 

Colour  Blindness.  —  There  are  three  fundamentally 
different  forms  of  congenital  colour  blindness,  namely, 
red-green  blindness,  yellow-blue  blindness,  and  total  colour 
blindness.  In  the  first  the  subject  confuses  reds  and 
greens.  This  is  by  far  the  commonest  form  of  colour  blind- 
ness, its  frequency  among  the  male  population  in  most 
European  countries  being  about  4  per  cent.  It  is  seldom 
met  with  in  women.  The  other  two  forms  of  colour  blind- 
ness are  far  rarer,  particularly  the  yellow-blue  form 
(exp.  64). 

There  are  two  important  classes  of  red-green  blindness 
In  the  one  the  spectrum  appears  of  the  same  length  as  in 
normal  vision.  In  the  other  the  red  end  of  the  spectrum 
appears  distinctly  darker  than  usual,  so  much  so  that  the 
extreme  red  is  not  visible  at  all.  These  two  classes  have 
been  respectively  called  the  "  photerythrous  "  and  "  scotery- 
throus  "  varieties.  They  appear  to  be  quite  distinct  from 
one  another,  no  intermediate  or  transitional  cases  being  met 
with. 

In  the  photerythrous  class  the  point  of  maximal  bright- 
ness of  the  spectrum  is  nearly  the  same  as  for  the  normal 
individual.  In  the  scoterythrous  class,  on  the  other  hand, 
it  lies  distinctly  nearer  the  green. 

In  mixing  the  red  end  of  the  spectrum  with  various 


84  EXPERIMENTAL  PSYCHOLOGY 

other  regions  to  produce  a  match  with  some  third  colour, 
the  two  classes  are  found  to  differ  very  distinctly  in  the 
amount  of  red  which  they  employ,  the  scoterythrous  requir- 
ing very  much  more  red  in  their  matches  than  the  photery- 
throus  class.  Further,  while  the  photerythrous  class 
closely  agrees  with  the  normal  individual  in  the  grey  which 
he  chooses  to  match  a  given  colour  in  the  most  peripheral 
("  colour-blind  ")  zone  of  the  retina,  to  the  scroterythrous 
class  the  colours  at  the  red  end  of  the  spectrum  appear  of 
an  abnormally  dark  grey  in  peripheral  vision. 

Both  classes,  however,  agree  in  seeing  in  the  green  of 
the  spectrum  a  "  neutral  band,"  that  is,  a  region  which  they 
match  with  a  grey.  They  likewise  match  the  comple- 
mentary colour  of  this  green  (a  bluish  red)  with  grey. 

A  red-green  blind  person  differs  from  the  normal  in 
that  for  him  may  be  found  two  (instead  of  three)  colours, 
by  combining  which  (with  black  and  white)  in  various  pro- 
portions, a  valid  match  may  be  obtained  with  any  other 
colour.  That  is  to  say,  the  diagram  corresponding  to  fig.  1 
is  for  him  not  a  triangle  but  a  straight  line. 

The  totally  colour-blind  individual  sees  the  spectrum  as 
a  colourless  band  differing  in  brightness.  There  appear  to 
be  at  least  two  classes  of  total  colour  blindness,  in  one  of 
which  the  region  of  maximal  brightness  is  in  the  yellow  as 
in  normal  vision,  while  in  the  other  the  region  of  maximal 
brightness  is  in  the  green.  In  many  cases  an  "  absolute 
central  scotoma  "  has  been  found  in  colour-blind  individuals 
of  the  latter  class ;  that  is  to  say,  they  are  totally  blind  at 
the  fovea. 

[Colour  blindness  may  be  acquired  owing  to  the  pro- 
longed action  of  coloured  light  on  the  retina,  or  owing  to 
the  influence  of  drugs  (e.g.  nicotin,  santonin)  or  of  disease. 
Acquired  colour  blindness  may  be  general  or  it  may  be 
confined  to  relatively  large  or  small  areas  of  the  retina. 
Usually  red  and  green  are  the  first  colours  to  be  lost. 
Santonin,  however,  produces  a  shortening  of  the  violet  end 


VISUAL  SENSATIONS  85 

of  the  spectrum,  while  objects  appear  violet  in  dark  and 
yellow  in  blue  light.  The  phenomena  of  acquired  colour 
blindness  differ  in  many  important  respects  from  those  of 
congenital  colour  blindness.  They  require  further  study 
before  they  can  be  employed  to  throw  light  on  the  nature  of 
the  latter  or  on  colour  vision  generally. 

Certain  people,  although  not  in  the  above  sense  colour 
blind,  have  "  anomalous  "  colour  vision.  They  may  differ 
from  normal  people  in  their  greater  susceptibility  to  adapta- 
tion and  contrast  with  respect  to  certain  colours ;  in  their 
hesitation  when  giving  a  name  to  certain  colours ;  in  re- 
quiring to  see  certain  colours  at  a  shorter  distance  from  the 
eye,  and  in  a  state  of  greater  saturation  and  intensity. 
Anomalous  colour  vision  may  be  revealed  when  such 
individuals  are  asked  to  match  mixtures  of  a  spectral 
red  and  yellowish  greens  with  a  spectral  yellow ;  some 
using  very  much  more  green,  others  more  red  than  normal 
individuals.] 

Flicker. — When  a  stimulus,  applied  to  a  given  retinal 
area,  is  repeated  with  adequate  frequency,  an  uninterrupted 
visual  sensation  is  produced.  The  sensation  corresponding 
to  each  individual  stimulus  always  outlasts  the  latter,  so 
that  when  the  intermittent  series  of  stimuli  is  sufficiently 
rapid,  a  fused  and  continuous  sensation  results.  Such 
fusion  may  be  studied  on  the  colour  wheel  (exp.  65).  [It  is 
found  that  within  certain  limits  the  speed  of  rotation, 
necessary  to  extinguish  nicker  and  to  produce  complete 
fusion,  varies  with  the  brightness  value  of  the  stimulus  and 
with  the  intensity  of  illumination.  The  brighter  a  colour 
sensation,  or  the  more  intense  a  colour  or  colourless  sensa- 
tion, the  greater  will  be  the  number  of  revolutions  (i.e.  the 
number  of  intermittent  stimuli)  necessary  to  produce  fusion. 
Colours  which  are  increased  in  brightness  value,  owing  to 
dark  adaptation  (page  87)  or  owing  to  contrast  (page  81), 
similarly  require  a  more  rapid  intermission  for  the  produc- 
tion of  fusion.  Observers  are  not  agreed  as  to  the  influence 


86  EXPERIMENTAL  PSYCHOLOGY 

of  the  relative  duration  of  the  periods  of  stimulation  and 
non-stimulation  upon  the  point  of  extinction  of  nicker. 

When  a  white  sector  upon  a  black  ground  is  very  slowly 
turned  on  the  colour  wheel,  a  series  of  black  bands  in  the 
form  of  radii  may  be  observed  on  that  part  of  the  white 
surface  which  first  stimulates  the  eye.  These  are  known, 
after  the  name  of  their  first  observer,  as  Charpentier's 
bands.  With  somewhat  more  rapid  rotation,  especially 
under  bright  illumination,  various  colours,  called  Fechner's 
colours,  may  be  visible  on  the  white  surface.  They  have 
been  attributed  to  the  unequal  action  of  white  light  on  the 
elementary  systems  of  cerebro-retinal  colour  apparatus,  so 
that  the  coloured  components  of  the  white  sensation  make 
their  appearance  at  different  moments. 

Two  kinds  of  flicker  can  be  roughly  distinguished,  before 
the  point  of  complete  fusion  is  reached, — a  "  coarse  "  and  a 
"  fine  "  flicker.  The  former  has  at  certain  speeds  a  glitter- 
ing character,  the  brightness  of  which  considerably  exceeds 
that  of  the  continuous  sensation.] 

When  flicker  is  once  extinguished,  further  increase  in 
the  rate  of  rotation  of  the  colour  wheel  produces  no  change 
in  the  character  of  the  sensation.  The  brightness  of  the 
fused  sensation,  upon  the  extinction  of  flicker,  is  (within 
certain  limits)  equal  to  the  total  brightness  of  the  individual 
stimuli,  if  it  be  imagined  that  this  total  brightness  is 
reduced  by  being  uniformly  distributed  over  the  periods  of 
excitation  and  non-excitation.  This  is  the  Talbot-Plateau 
law  (exp.  66). 

[Determinations  of  Brightness. — The  brightness  of  a 
colour  sensation  may  be  determined  by  comparing  it  with 
a  colourless  sensation.  The  comparison  may  be  made  (i.) 
directly,  (ii.)  at  the  periphery  of  the  retina,  or  (iii.)  by  the 
flicker  method.  There  are  other  modes  of  determining 
brightness,  but  the  above  yield  results  which  are  fairly 
consistent  with  jone  another  (exps.  47,  67,  68).] 

The    Intrinsic   Light    of  the   Retina. — The    visual    ex- 


VISUAL  SENSATIONS  87 

perience  that  is  obtained  when  the  retina  has  been  for 
a  short  time  completely  shielded  from  external  stimulation 
is  very  far  from  being  identical  with  that  of  blackness.  A 
greyish  light  is  seen,  which  has  been  termed  "  the  intrinsic 
light  of  the  retina."  We  shall  later  (page  106)  have  reason 
to  question  the  appropriateness  of  the  name.  Several 
writers  have  given  full  accounts  of  the  intrinsic  light, 
with  slight  individual  variations.  But  as  it  is  so  easy  for 
every  one  to  experience  and  to  study  the  condition  himself 
(exp.  69),  a  detailed  description  is  unnecessary  here. 

Purkinje's  Phenomenon.  Rod  Vision.  —  The  different 
brightness  of  colours  and  their  photochromatic  intervals 
(page  78)  are  closely  connected  with  one  another.  If  an 
observer,  standing  in  a  dimly  lighted  room,  regard  several 
transparent  patches  of  different  colour,  which  are  placed  on 
one  of  the  walls,  and  if  these  colours  be  illuminated  by  light 
transmitted  through  apertures  in  the  wall  from  an  adjoining 
room,  he  will  notice  that  the  relative  brightness  values  of 
the  different  colours  change,  as  the  intensity  of  the  colours 
is  decreased  by  diminishing  the  transmitted  illumination. 
The  colours  belonging  to  the  red  end  of  the  spectrum  will 
appear  relatively  darker,  those  of  the  blue  end  brighter, 
while  the  region  of  maximum  brightness  will  pass  from  the 
yellow  to  the  green.  This  change  of  relative  brightness  is 
known,  after  the  name  of  its  discoverer,  as  "  Purkinje's 
phenomenon."  If  the  colours  be  still  further  reduced  in 
intensity,  they  lose  their  hue  and  finally  give  rise  merely  to 
colourless  sensations.  In  this  stage,  Purkinje's  phenomenon 
is  most  marked ;  colours  at  the  extreme  red  end  being  so 
dark  that  they  are  invisible,  while  the  region  of  maximal 
brightness  is  in  the  green  (exps.  69,  70). 

If,  however,  the  observer  stands  in  a  brightly  lighted 
room  while  the  intensity  of  the  colours,  illuminated  by  light 
that  is  transmitted  through  the  wall,  is  being  gradually 
changed  as  before,  neither  Purkinje's  phenomenon  nor  the 
photochromatic  interval  is  to  be  observed.  The  colours 


88  EXPERIMENTAL  PSYCHOLOGY 

retain  their  hue  and  preserve  their  relative  brightness  to 
one  another  until  they  are  too  faint  to  give  rise  to  any 
experience  but  black.  Thus  Purkinje's  phenomenon  and 
the  photochromatic  interval  depend  not  merely  on  the 
intensity  of  the  colour  stimulus,  but  also  on  the  condition  of 
adaptation  of  the  retina.  They  are  absent  in  a  retina 
exposed  and  hence  adapted  to  bright  light ;  they  are  to  be 
obtained  only  from  the  dark-adapted  eye.  That  this 
adaptation  is  due  to  changes  in  the  retina  and  not  to 
changes  in  the  size  of  the  pupil  may  be  proved  in  various 
ways ;  for  example,  by  the  persistence  of  Purkinje's  pheno- 
menon and  the  photochromatic  interval  when  the  pupillary 
muscles  have  been  paralysed  by  atropin,  and  by  the  fact 
that  Purkinje's  phenomenon  and  the  photochromatic  interval 
are  absent  when  the  area  of  retinal  excitation  by  the 
coloured  areas  is  limited  to  the  fovea  (exp.  71). 

The  absence  of  Purkinje's  phenomenon  and  of  the  photo- 
chromatic  interval  at  the  fovea,  when  taken  in  conjunction 
with  the  absence  of  rods  at  the  fovea,  suggests  that  while 
the  cones  are  concerned  with  ordinary  vision  under  con- 
ditions of  bright  adaptation,  it  is  the  function  of  the  rods  to 
develop  colourless  sensations  in  the  dark-adapted  eye.  On 
this  supposition,  the  rods  become  the  end  organs  of  colour- 
less vision  for  dim  light. 

Now  it  is  interesting  to  note  that  the  visual  purple, 
which  the  rods  contain,  is  bleached  most  rapidly  by  green 
but  is  relatively  unaffected  by  red  light;  that  this  visual 
purple  is  bleached  far  more  speedily  than  it  is  regenerated ; 
and  that  the  bleaching  in  one  eye  is  found  to  have  no  effect 
on  the  condition  of  the  visual  purple  in  the  other  eye. 
Corresponding  with  these  observations,  we  find  that  green 
gives  rise  to  the  brightest,  red  to  the  darkest  colourless 
sensation  in  the  dark-adapted  eye ;  that  change  of  adapta- 
tion from  darkness  to  brightness  is  immeasurably  more 
rapid  than  the  converse  change;  and  that  changes  in  the 
adaptation  of  one  eye  have  no  effect  on  the  state  of  adapta- 


VISUAL  SENSATIONS  89 

tion  of  the  other  eye.  Further,  the  rods  and  the  visual 
purple  are  especially  developed  in  almost  all  animals  that 
live  underground  or  are  nocturnally  active. 

We  are  thus  able  to  account  for  that  class  of  total 
colour  blindness  (page  84)  in  which  an  absolute  central 
scotoma  is  present,  and  the  different  parts  of  the  spectrum 
appear  of  the  same  relative  brightness  as  in  the  case  of  the 
normal  dark-adapted  eye.  In  these  cases  there  is  almost 
invariably  a  well-marked  intolerance  of  bright  light  (photo- 
phobia), and  usually  there  are  irregular  oscillatory  move- 
ments of  the  eyes  (nystagmus).  All  four  defects,  the  total 
colour  blindness,  the  central  scotoma,  the  photophobia,  and 
the  nystagmus,  may  be  ascribed  to  the  inaction  of  the  cone 
apparatus.  The  individual  relies  solely  upon  the  rods  which 
are  adapted  for  use  in  twilight,  and  are  absent  at  the 
fovea,  the  usual  region  for  most  favourable  vision  and  steady 
regard. 

[The  After-effects  of  Momentary  Colour  Stimuli.  The 
Positive  After-image. — This  view,  that  the  rods  of  the  retina 
are  the  end  organs  concerned  in  twilight  vision,  and  that 
they  develop  only  colourless  sensations,  was  first  put 
forward  by  Schultze,  and  has  been  since  developed  to  its 
present  form  chiefly  by  Konig  and  von  Kries.  The  theory 
gains  striking  support  from  observations  on  the  after-effect 
of  momentary  colour  stimuli.  When  the  eye  is  adapted  to 
dim  light,  a  momentary  colour  stimulus  produces  a  single 
colour  sensation  which  is  attended  by  a  succession  of  after- 
effects, namely,  a  series  of  fluctuating  after-sensations  of  the 
same  hue  as  the  stimulus,  followed  by  a  series  of  fluctuating 
colourless  after  -  sensations.  All  these  after  -  sensations 
fluctuate  in  the  sense  that  each  waxes  and  wanes,  rising 
to  a  maximum  and  then  declining,  the  maximal  rise  being 
less  and  less  for  successive  members  of  the  series.  The 
coloured  after-sensations  are  doubtless  due  to  the  persistent 
activity  of  the  cones  after  the  stimulus  has  been  removed. 
The  colourless  after-sensations  are  clearly  due  to  a  similar, 


90  EXPERIMENTAL  PSYCHOLOGY 

but  more  sluggish,  after-action  on  the  part  of  the  rods,  since 
they  are  absent  at  the  fovea,  are  brightest  with  a  green 
stimulus,  and  are  so  dark  as  to  be  absent  with  a  red 
stimulus. 

These  fluctuating  after-sensations  are  followed  by  a 
continuous  steady  after-sensation,  which  is  grey  in  the  case 
of  the  dark-adapted  eye  (save  with  red  and  at  the  fovea, 
when  it  is  absent),  but  which  to  the  bright-adapted  eye 
appears  in  the  same  hue  as  the  original  sensation.  This  is 
known  as  the  "  positive  after-image,"  and  is  generally 
attributed  to  a  persistent  activity  of  the  once  excited 
retinal  area. 

The  separate  after-effects  of  the  cones  and  rods,  which 
have  just  been  described,  require  special  apparatus  and 
experience  for  satisfactory  observation.  But  the  more 
steady  and  lasting  positive  after-image  may  be  observed 
without  difficulty  (exp.  72). 

When  the  eyes  are  closed  or  placed  in  darkness,  after 
they  have  fixated,  for  not  too  long  a  period,  a  white  patch, 
a  positive  after-image  is  obtainable,  provided  that  the  back- 
ground on  which  the  patch  has  rested  be  not  too  dark,  and 
that  the  margins  of  the  surface  be  not  too  sharply  defined. 
If  these  conditions  are  not  fulfilled,  a  halo  surrounds  the 
after-image,  and  the  latter  is  dark  or  "negative"  in 
character  (exp.  54).  Moreover,  the  after-image  of  a  bright 
object,  e.g.  the  sun,  becomes  positive  or  negative,  according 
as  the  ground  on  to  which  it  is  projected  is  darker  or 
brighter  than  itself.  A  similar  reversal  is  said  to  occur  in 
the  case  of  coloured  after-images.  Under  certain  conditions 
they  appear  to  be  positive  or  negative  according  as  they  are 
projected  on  to  a  black  or  on  to  a  white  (or  grey)  back- 
ground. Thus  we  see  that  a  very  close  relation  exists 
between  positive  and  negative  after-effects.  But  further 
experimental  evidence  is  necessary  before  we  can  determine 
this  relation  more  precisely. 

Before    a    coloured  or  colourless  positive  after-image 


VISUAL  SENSATIONS  91 

disappears,  it  often  passes  through  different  colours.  This 
play  or  "  flight "  of  colours  is  doubtless  due  to  the  complex 
nature  of  our  visual  sensations,  which  are  each  the  resultant 
of  more  elementary  processes.  The  after-effects  of  these 
components  vary  in  duration,  and  hence  produce  the 
coloured  waning  of  the  positive  after-image  (exp.  72).] 


BIBLIOGRAPHY. 

H.  von  Helmholtz,  Handbuch  d.  physiologiachen  Optik,  2te  Aufl., 
Hamburg,  1885-96.  E.  Bering,  Zur  Lehre  vom  Lichtsinne  [1872-4],  Wien, 
1878  ;  Grundziige  der  Lehre  vom  Lichtsinn  (offprinted  from  the  Handbuch  d. 
Augenheilkunde,  2te  Aufl.),  Leipzig,  1905,  in  course  of  publication.  [A 
fairly  complete  bibliography  of  Hering's  papers  on  visual  sensation  is  given 
in  Sanford's  Experimental  Psychology,  Boston,  1897,  179,  180].  Lord 
Rayleigh,  "Experiments  on  Colour,"  Nature,  1881,  xxiv.  264  ;  1882,  xxv. 
64.  A.  Charpentier,  La  lumitre  et  les  couleurs,  Paris,  1888.  J.  von  Kries, 
Vber  d.  Funktion  d.  Netzhautstabehen,  Ztschr.  f.  Psychol.  u.  Physiol.  d. 
Sinnesorgane,  1895,  ix.  81  ;  Theoretische  Studien  iiber  d.  Umstimmung  d. 
Sehorgans,  Freiburg,  1902  ;  "Die  Gesiclitsempfindungen "  in  Nagel's  Hand- 
buch d.  Physiologic  d.  Menschen,  Braunschweig,  1904,  iii.  109.  Shelford 
Bidwell,  "Oil  Subjective  Colour  Phenomena  attending  Sudden  Changes  of 
Illumination,"  Proc.  Roy.  Soc.  Lond.,  1896,  Ix.  368  ;  "On  Negative  After- 
images, and  their  Relation  to  certain  other  Visual  Phenomena,"  ibid.,  1901, 
Ixviii.  262.  C.  S.  Sherrington,  "On  the  Reciprocal  Action  in  the  Retina," 
etc.,  Journ.  of  Physiol.,  1897,  xxi.  33.  W.  H.  R.  Rivers,  "Vision,"  in  Schafer's 
Text-Book  of  Physiology,  Edinburgh  and  London,  1900,  ii.  1026.  A. 
Tschermak,  "Uber  Kontrast  und  Irradiation,"  in  Asher  and  Spiro's  Ergcl- 
nissed.  Physiologic,  Wiesbaden,  1903,  Abt.  2,  ii.  726.  W.  Wirth,  "Foit- 
schritte  auf  d.  Gebiet  d.  Psychophysik  d.  Licht-  u.  FarbenempfiuduDg," 
Arch.f.  d.  ges.  Psychol.  (Literaturbericht),  1903,  i.  21  ;  1905,  v.  1,  77,  149. 
W.  M'Dougall,  "The  Sensations  excited  by  a  Single  Momentary  Stimula- 
tion of  the  Eye,"  Brit.  Journ.  of  Psychol.,  1904,  i.  78.  J.  W.  Baird,  The 
Colour  Sensitivity  of  the  Peripheral  Retina,  Washington,  1905. 


CHAPTEK  VII 
ON  VISUAL  SENSATIONS1  (concluded) 

The  Young -Helmlioltz  Theory  of  Colour  Vision. — The 
Young-Helmholtz  theory,  first  proposed  by  Thomas  Young 
and  later  advanced  by  Clerk  Maxwell  and  especially  by  Helm- 
hoi  tz,  rests  on  the  sufficiency  of  three  standard  colours, 
variously  combined,  to  produce  colourless  and  all  colour 
sensations  (page  78).  It  was  first  supposed  that  three 
distinct  sets  of  nerve  fibres  exist,  each  of  which  is  specially 
sensitive  to  waves  of  a  certain  length.  But  in  the  more 
modern  form  of  the  theory,  the  three  sets  of  nerve  fibres 
are  usually  replaced  by  three  photochemical  substances  ; 
and  it  is  supposed  that  the  first  apparatus 2  is  most  sensitive 
to  a  carmine  red,  i.e.  a  red  bluer  than  the  extreme  red  of 
the  spectrum,  that  the  second  is  most  sensitive  to  a  slightly 
yellowish  green,  and  that  the  third  apparatus  is  most 
sensitive  to  an  ultramarine  blue.  All  colour  stimuli  are 
considered  to  act  on  all  three  systems  of  apparatus,  but  in 
different  degrees ;  red  colours  acting  most  on  the  first,  least 
on  the  last,  blue  colours  acting  most  on  the  last  and  least 
on  the  first,  while  both  these  colours  act,  less  strongly  than 
green,  on  the  second  apparatus.  The  sensation  produced 
depends  on  the  relative  extent  to  which  the  three  systems 
of  apparatus  are  stimulated.  When  they  are  all  three 

1  See  footiiote  to  Chapter  III. 

2  It  must  be  understood  that  no  evidence  is  available  for  determining  the 
seat  of  the  various  processes,  or  of  the  various  systems  of  apparatus,  which 
have  been  posited  in  the  different  theories  of  colour  vision.     The  processes 
may  be  partly  of  retinal  and  partly  of  more  central,  e.g.  subcortical,  origin. 


VISUAL  SENSATIONS  93 

highly  stimulated,  the  sensation  of  white  results.  When 
they  are  stimulated  in  other  proportions,  other  sensations, 
e.g.  sensations  of  yellow  and  violet,  result.  The  theory  is 
thus  the  result  of  applying  to  the  cerebro-retinal  apparatus 
the  features  embodied  in  the  colour  triangle  (fig.  1). 

Complementary  after  -  sensations  were  attributed  by 
Fechner  and  Helmholtz  to  fatigue,  their  colour  being  due 
to  the  subsequent  over-action  of  the  apparatus  which, 
during  the  application  of  the  stimulus,  had  received  a 
weaker  stimulus.  According  to  this  explanation,  a  grey 
ground  is  seen  as  green  after  fixation  of  a  red  surface, 
because,  the  red  apparatus  being  most  fatigued,  the  grey 
stimulus  excites  more  powerfully  the  remaining  two  systems 
of  the  colour  apparatus.  When  the  coloured  after-image 
appears  during  closure  of  the  eyes,  the  result  is  similarly 
attributed  to  the  affection  of  the  intrinsic  light  of  the 
retina  (page  86). 

The  effects  of  simultaneous  contrast  received  at  Helm- 
holtz's  hands  the  following  strained  psychological  explana- 
tion. A  grey  patch  on  a  green  surface  becomes  tinged  with 
rose,  because,  imagining  that  a  part  of  the  green  surface  is 
transparent  so  that  the  grey  is  seen  through  the  green,  the 
observer  knows  that  a  colour  must  be  reddish  in  order  that 
it  may  appear  grey  when  transmitted  through  green. 
This  knowledge  is  supposed  to  have  been  based  upon 
previous  experience,  and  to  be  applied  unconsciously  to  the 
conditions  of  the  experiment. 

The  scoterythrous  class  of  red-green  blindness  was 
attributed  by  Helmholtz  to  absence  of  the  red  apparatus, 
the  photerythrous  class  to  absence  of  the  green;  and  the 
rare  cases  of  yellow-blue  blindness  have  been  held  to  depend 
on  absence  of  the  blue  apparatus. 

Hering's  Theory  of  Colour  Vision. — Bering's  theory  is 
based  (i.)  upon  the  seemingly  "  elementary  "  nature  of  red, 
yellow,  green,  blue,  white,  and  black,  when  all  the  possible 
visual  sensations  are  carefully  considered  by  introspection, 


94  EXPERIMENTAL  PSYCHOLOGY 

and  (ii.)  upon  the  relation  of  the  complementary  colours  to 
one  another.  He  assumes  that  there  are  two  elementary 
systems,  one  of  which  gives  rise  to  red  and  green,  the  other 
to  yellow  and  blue  sensations,  and  that  there  is  a  third 
apparatus,  excitation  of  which  gives  rise  to  the  colourless 
series  of  sensations.  According  to  this  theory,  the  physio- 
logical actions  of  a  colour  stimulus  and  of  its  complementary 
colour  stimulus  are  antagonistic.  Eed,  for  example,  causes 
a  katabolic  (or  dissimilation)  change  in  the  red-green 
apparatus,  yellow  a  like  change  in  the  yellow-blue  apparatus; 
green  causes  an  anabolic  (or  assimilation)  change  in  the 
former  apparatus,  blue  a  like  change  in  the  latter.  The 
sensation  of  orange  results  from  katabolism  in  the  red-green 
and  the  yellow-blue  apparatus,  that  of  purple  from  kata- 
bolism in  the  former  combined  with  anabolism  in  the  latter 
apparatus. 

Each  apparatus  always  tends  to  recover  equilibrium, 
upon  the  removal  of  the  stimulus.  After  the  red-green 
apparatus  has  been  made  to  undergo  dissimilation  owing 
to  the  "  allonomous "  action  of  a  red  stimulus,  it  proceeds 
to  return  to  equilibrium  by  an  "autonomous"  process  of 
assimilation,  thereby  developing  the  complementary  after- 
sensation  of  green.  Thus  one  colour  sensation  automatically 
evokes  the  opposite  or  complementary  after-sensation. 

Bering  explains  the  effects  of  simultaneous  contrast 
by  a  similar  principle.  He  supposes  that  the  process  of 
assimilation,  set  up  by  a  stimulus  in  one  part  of  the  retina, 
brings  about  a  process  of  dissimilation  of  the  same  apparatus 
in  neighbouring  parts  of  the  retina. 

According  to  Hering,  the  series  of  grey  sensations  are 
not  to  be  considered  (as  they  are  in  the  Young-Helmholtz 
theory)  merely  as  different  stages  in  the  intensity  of  excita- 
tion of  one  and  the  same  sensation,  white.  Hering  insists 
that,  introspectively,  greys  have  a  two-dimensional  (a  black- 
white)  relation,  being  compounded  of  two  variables,  pure 
black  and  white,  in  different  ratios. 


VISUAL  SENSATIONS  95 

He  insists  that  black  is  a  positive  sensation  no  less  than 
white.  And  he  points  to  the  intrinsic  light  of  the  retina 
(page  86)  as  evidence  that  blackness  is  not  the  condition 
of  an  absence  of  retinal  stimulation.  He  calls  the  brightness 
of  the  intrinsic  light  the  "  sensation  of  mean  grey."  In 
this  condition  of  equilibrium,  the  white-black  apparatus 
may  be  stimulated  to  undergo  dissimilation  and  to  produce 
a  brighter  grey  or  a  white  sensation  ;  and,  after  the  removal 
of  the  stimulus,  it  will  undergo  assimilation,  developing  a 
darker  grey  or  a  black  after-sensation.  But  in  such  a  con- 
dition of  the  white-black  apparatus,  it  can  only  be  stimulated 
to  undergo  assimilation  owing  to  successive  or  simul- 
taneous contrast. 

[Hering  attributes  the  bright  halo,  which  under  certain 
conditions  surrounds  complementary  after-images  (exps. 
54,  74),  to  a  process  of  "  successive  induction."  When,  for 
example,  a  white  square,  upon  a  black  background  has  been 
for  some  time  fixated,  the  assimilation  process,  which, 
according  to  Hering,  has  been  especially  active  just  beyond 
the  contrasting  edges  of  the  white  square,  reaches  such  a 
height  that  finally  a  process  of  "  simultaneous  induction  " 
is  set  up ;  assimilation  gives  way  to  dissimilation.  When 
now  the  eyes  are  turned  away  from  the  white  square,  this 
dissimilation  process  continues.  It  is  owing  to  this  suc- 
cessive induction  that  a  bright  halo  appears  around  the  dark 
after-image  of  the  white  surface.  And  the  dark  after-image 
results  from  the  contrasting  assimilation  process,  which  is 
evoked  by  the  dissimilation  process  that  produces  the  halo. 

The  dark  halo,  which  under  other  conditions  surrounds 
an  after-image,  is  attributed  by  Hering  to  the  effect  of 
simultaneous  contrast.  Thus,  when  a  black  square  has 
been  fixated  upon  a  white  background,  the  bright  after- 
image of  the  former  is  produced  by  a  subsequent  process 
of  assimilation  which  evokes  a  simultaneous  dissimilation 
process  in  the  adjoining  retinal  region. 

Hering  ascribes  the  positive   after-image,  produced  by 


96  EXPERIMENTAL  PSYCHOLOGY 

closing  the  eyes  after  gazing  at  a  bright  object  (e.g.  the  sun), 
to  exhaustion  of  assimilation  material  during  fixation.  In 
consequence  of  this,  no  assimilation  is  subsequently  possible, 
and  all  that  remains  is  a  feeble  process  of  dissimilation  due 
to  internal  stimulation.] 

Hering  distinguishes  three  states  of  equilibrium,  which 
the  antagonistic  processes  of  assimilation  and  dissimilation 
may  attain.  The  first  occurs  in  the  resting  eye,  and  it  is 
called  "  autonomous  equilibrium  at  mean  potential.3'  It  is 
during  the  return  to  this  neutral  condition  that  complement- 
ary after-sensations  are  produced  when  the  eyes  are  closed. 
The  other  two  states  of  equilibrium  are  brought  about  by 
adaptation  to  a  continuous  stimulus  (page  79  and  exp.  74). 
The  assimilation,  produced,  for  example,  by  a  constant  blue 
stimulus,  becomes  so  reduced  that  it  is  ultimately  met  by  an 
equal  degree  of  dissimilation  change  in  the  same  apparatus. 
Hering  terms  this  condition  of  adaptation  "allonomous 
equilibrium  at  high  potential/'  The  expression  "  allonom- 
ous equilibrium  at  low  potential"  is  similarly  applied  to 
the  condition  of  adaptation,  produced  by  a  continuous  red 
or  yellow  stimulus,  the  dissimilatory  action  of  which  is 
supposed  to  be  ultimately  counterbalanced  by  an  equivalent 
assimilatory  action  in  the  same  apparatus.  When  under 
these  conditions  the  stimulus  is  removed,  the  complementary 
sensation  develops,  owing  to  a  return  from  allonomous  to 
autonomous  equilibrium. 

Hering  considers  that  the  state  of  normal  adaptation  of 
the  eye  is  the  direct  result  of  allonomous  equilibrium ;  the 
white-black  apparatus,  for  example,  is  continually  being 
acted  upon  throughout  the  day.  Dissimilation  is  most 
active  in  sunlight,  less  active  indoors.  But  in  each  con- 
dition the  apparatus  soon  attains  a  condition  of  allonomous 
equilibrium,  at  low  or  high  potential,  owing  to  the  ultimately 
compensating  effect  of  the  opposite  assimilatory  process. 

Hering  supposes  that  the  red-green  apparatus  is  missing 
in  red-green  blindness,  the  yellow-blue  apparatus  in  yellow- 


VISUAL  SENSATIONS  97 

blue  blindness,  while  in  total  colour  blindness  the  white- 
black  apparatus  alone  remains.  He  attributes  the  vision 
characteristic  of  the  peripheral  retina  to  similar  conditions. 

He  supposes  that  the  brightness  of  a  colour  sensation  is 
due,  partly  to  the  action  of  the  colour  stimulus  on  the 
white- black  apparatus,  and  partly  to  the  "  intrinsic "  or 
"  specific "  brightness  contributed  in  different  degrees  by 
the  two  systems  of  colour  apparatus.  Under  ordinary 
conditions,  all  colour  stimuli  act  not  only  on  one  (or  more 
usually  on  both)  of  the  two  systems  of  colour  apparatus,  but 
also  on  the  black-white  apparatus.  The  condition  of  dark- 
adaptation,  however,  permits  of  little  or  no  "  specific " 
contribution  towards  brightness  from  either  system  of 
colour  apparatus.  Under  these  circumstances,  only  the 
white-black  apparatus,  which  is  now  in  a  condition  of 
equilibrium  at  high  potential,  is  stimulated.  On  the  other 
hand,  with  increasing  illumination,  the  anabolically  active 
colours,  red  and  yellow,  contribute  more  and  more  positively, 
the  green  and  blue  more  and  more  negatively,  to  the  total 
brightness  value,  owing  to  their  specific  action  on  the 
colour  apparatus.  The  theory  attempts  in  this  way  to 
explain  the  Purkinje  phenomena. 

The  four  "  fundamental "  colours,  chosen  by  Hering,  are 
the  purest  red,  yellow,  green,  and  blue,  which  can  be 
attained  by  introspection.  He  points  out  that,  in  reality, 
spectral  red  appears  yellowish  to  an  observer,  and  that  the 
truest  red  is  obtained  by  mixing  with  the  former  a  small 
amount  of  blue  light.  According  to  Hering,  spectral  red 
has  a  not  inconsiderable  action  on  the  yellow-blue  apparatus. 
And  a  mixture  of  spectral  red  and  green  produces  the 
sensation  of  yellow,  because,  although  the  red  and  green 
processes  neutralise  one  another,  the  effect  of  the  spectral 
red  stimulus  on  the  yellow-blue  apparatus  remains. 

The  fundamental  colours  thus  obtained  are,  significantly, 
found  by  Hering  to  be  precisely  those  which,  when  passed 
from  the  periphery  to  the  centre  of  the  retina,  yield  sensa- 
7 


98  EXPERIMENTAL  PSYCHOLOGY 

tions  that  undergo  no  alteration  in  hue  (exp.  52).  The 
fundamental  red  and  blue  agree  with  the  recent  determina- 
tions made  by  adherents  of  the  Young-Helmholtz  three- 
colour  theory ;  the  green  in  the  latter  being  somewhere 
intermediate  between  Hering's  fundamental  yellow  and 
green. 

Criticism  of  these  Theories. — At  the  present  time  the 
majority  of  those  who  hold  to  a  three-colour  theory  admit 
the  impossibility  of  retaining  Helmholtz's  suppositions  that 
complementary  after-sensations  are  due  to  fatigue ;  that 
the  effects  of  simultaneous  contrast  are  of  purely  psycho- 
logical origin ;  and  that  partial  colour  blindness  is  due  to 
the  mere  absence  of  one  or  other  of  the  three  systems  of 
apparatus. 

Against  the  fatigue  hypothesis  of  complementary  after- 
sensations  the  following  objections  may  be  ranged. 
Complementary  after-sensations  may  be  obtained  after 
extremely  short  periods  of  fixation,  and  are  as  vivid  in 
young  subjects,  or  after  a  night's  rest,  as  in  the  old,  or  after 
the  day's  fatigue.  Further,  they  are  surprisingly  bright 
during  closure  of  the  eyes  after  fixation,  when  they  are 
seen  merely  against  the  grey  ground  of  the  intrinsic  light  of 
the  retina.  To  explain  why  the  brightness  of  a  part  of  the 
intrinsic  light  should  be  greater  than  that  of  the  whole, 
Helmholtz  once  again  had  recourse  to  psychological  factors ; 
holding  that  the  image  appears  so  bright,  because,  in  the 
absence  of  any  basis  of  comparison,  we  judge  the  general 
light  of  the  retinal  field  to  be  unduly  dark.  Lastly,  we  are 
able  to  obtain  after-sensations,  which  are  equal  or  superior 
in  saturation  and  brightness  to  a  sensation  of  like  hue 
(exp.  56). 

Any  psychological  explanation  of  simultaneous  contrast 
must  obviously  be  of  a  most  unsatisfactory  nature.  It  is 
sufficient  to  point  out  that  brightness  contrast  affects  the 
point  of  extinction  of  flicker  (page  85),  and  that  in  experi- 
ments upon  binocular  colour  contrast,  two  differently 


VISUAL  SENSATIONS  99 

coloured  fields,  seen  by  separate  eyes  simultaneously,  induce 
different  contrast  colours  in  the  two  eyes  (page  281).  In  the 
latter  case,  one  can  hardly  suppose  that  unconscious  infer- 
ences from  previous  experience  can  be  carried  so  far  as  to 
lead  simultaneously  to  two  different  errors.  On  the  other 
hand,  it  is  clear  that  psychological  influences  cannot  be 
altogether  neglected  (exp.  58).  They  appear  to  be  of  the 
same  nature  as  those,  arising  from  previous  experience,  to 
which  we  have  already  drawn  attention  in  the  preceding 
chapter. 

There  are  good  reasons  for  disbelieving  that  partial 
colour  blindness  is  due  merely  to  the  absence  of  one  or 
other  of  the  three  systems  of  colour  apparatus.  For  it  is 
generally  recognised  that  the  white  sensations  of  the  red- 
green  blind  are  identical  with  those  of  the  normal  eye. 
Again,  we  have  evidence  which  goes  to  show  that  the  colour 
sensations  of  the  red-green  blind  correspond  to  the  yellow 
and  blue  of  the  normal  eye.  A  case  of  unilateral  red-green 
colour  blindness  is  on  record,  in  which  the  colour  sensations 
of  the  affected  eye  were  found  to  correspond  to  the  yellow 
and  blue  of  the  normal  eye.  We  are  therefore  compelled, 
if  we  retain  the  Young-Helmholtz  theory,  to  give  up  the 
idea  of  the  absence  of  one  of  the  components  in  partial 
colour  blindness.  We  must  suppose  that  partial  colour 
blindness  is  due  to  abnormal  changes  either  in  the  relative 
sensitivity  of  the  three  photochemical  substances  to  rays  of 
different  wave  length,  or  in  more  central  processes  initiated 
by  the  excitation  of  these  substances. 

Those  who  uphold  the  trichromic  (three-colour)  theory  of 
Young-Helmholtz  at  the  present  day  fully  realise  these 
difficulties,  and  beside  it  admit  a  number  of  auxiliary 
theories.  For  example,  they  rightly  feel  it  impossible  any 
longer  to  deny  that  colourless  sensations,  under  conditions 
of  dark-adaptation  of  the  retina,  are  the  expression  of 
activity  in  an  altogether  distinct  apparatus. 

[Rollet  has  suggested,  and  in  this  country  McDougall  has 


ioo  EXPERIMENTAL  PSYCHOLOGY 

independently  elaborated,  a  theory  of  simultaneous  contrast, 
in  order  to  escape  from  the  difficulties  involved  in  Helmholtz's 
explanation.  This  theory  attributes  simultaneous  contrast 
to  the  inhibitory  action  of  a  given  cortical  visual  process 
upon  the  visual  processes  in  neighbouring  cortical  regions. 
If,  for  example,  a  grey  surface  be  fixated  which  has  a  red 
square  upon  it,  the  cortical  area  excited  by  the  red  only 
differs  in  activity  from  that  excited  by  the  grey  (according 
to  Helmholtz)  in  that  the  red  apparatus  is  far  more  power- 
fully stimulated  in  the  former.  The  theory,  now  introduced, 
supposes  that  the  highly  excited  activity  of  this  "  red  area  " 
depresses  the  activity  of  the  red  apparatus  in  the  neighbour- 
ing "  grey  area  "  of  the  cortex,  whereby  the  blue  and  green 
in  the  latter  area  predominate  over  the  red  apparatus. 
McDougall  supposes  that  such  inhibition  is  due  to  a  drainage 
of  nervous  energy  from  less  active  (or  inactive)  into  more 
active  regions, — a  hypothesis  which  needs  stronger  physio- 
logical support  before  it  can  be  accepted.  We  shall  again 
mention  it  (page  320)  when  we  come  to  deal  with  the  process 
of  attention.  It  is  owing  to  this  supposed  inhibition  of  the 
red  that  the  contrast  blue-green  colour  is  produced. 

McDougall  further  suggests  that  all  after-images  are 
primarily  due  to  the  decomposition  of  certain  retinal  mother 
substances  by  the  light  rays,  and  to  the  persistence  and 
stimulating  effect  of  the  unknown  substances  resulting  from 
such  decomposition.  He  supposes  that  the  retinal  nerve 
endings,  excited  by  these  latter  substances,  differ  in 
sensitivity  under  different  conditions  and  in  different  areas 
of  the  retina,  and  that  fatigue  may  occur  not  only  in  these 
nerve  endings,  but  in  the  corresponding  cortical  regions. 
On  the  basis  of  these  suppositions,  he  has  established  a  very 
complex  theory,  which,  he  believes,  can  satisfactorily  account 
for  the  peculiar  relations  between  positive  and  negative 
after-images,  indicated  in  his  numerous  experiments.  It 
may  be  added  that  he  attributes  the  phenomena  of  simul- 
taneous induction  (page  81)  to  the  diffusion  of  the  above- 


VISUAL  SENSATIONS  101 

mentioned  stimulating  products  of  retinal  'decomposition 
into  neighbouring  retinal  areas.] 

Unsatisfactory  as  is  the  Young-Helmholtz  theory  in  its 
original  form,  it  must  at  the  same  time  be  admitted  that 
Bering's  theory  is  by  no  means  free  from  difficulties.  There 
are  many  who  find  it  inconceivable  that  a  sensory  experi- 
ence should  be  produced  by  assimilation  changes  in  living 
substance.  Even  those  who  accept  Hering's  views  as  to  the 
elementary  nature  of  the  sensations  yellow  and  white,  and 
as  to  the  relation  of  colourless  and  colour  sensations  under 
ordinary  conditions  of  illumination,  feel  it  impossible  to 
reject  von  Kries's  theory  of  rod  vision  in  the  dark-adapted 
eye;  they  have  had  either  to  discard  or  to  consider  as 
inadequate  Hering's  theory  of  the  specific  brightness  of 
colours. 

Others,  again,  refuse  on  the  following  ground  to  place 
the  black-white  series  of  sensations  on  the  same  physio- 
logical basis  as  the  red-green  or  the  yellow-blue.  Between 
the  black  and  white  there  exists  every  transitional  shade  of 
grey,  whereas  between  the  yellow  and  blue  or  between  the 
red  and  green  obviously  a  very  different  relation  subsists. 
When  the  red-green  or  the  yellow-blue  apparatus  is  in 
equilibrium,  no  colour  sensation  results,  but  when  the 
white-black  apparatus  is  in  equilibrium,  a  sensation  of 
grey  occurs. 

The  shortening  of  the  red  end  of  the  spectrum  in  the 
scoterythrous  form  of  red-green  blindness  was  ascribed  by 
Hering  to  unusually  slight  pigmentation  of  the  macula  and 
of  the  lens.  For  various  reasons,  however,  this  explanation 
is  unsatisfactory ;  partly  owing  to  the  difficulty  of  account- 
ing for  the  absence  of  cases  of  colour  blindness  which  are 
intermediate  in  degree  between  the  scoterythrous  and  the 
photerythrous  varieties. 

Hering's  explanation  of  positive  after-images  is  like- 
wise very  far  from  satisfactory.  We  have  already  drawn 
attention  (page  90)  to  the  possibility  of  reversing  a  positive 


102  EXPERIMENTAL  PSYCHOLOGY 


after-image  (i.e.  making  it  negative),  if  its  brightness  be 
less  than  that  of  the  background  on  which  it  is  pro- 
jected. 

[Contrast  in  a  Smoothly  Graded  Disc. — It  is  not  easy  to 
see  how  Bering's  explanation  of  simultaneous  contrast  can 
account  for  the  effects  observed  when  a  star  of  white  paper, 
mounted  on  a  larger  black  disc  (fig.  2),  is  rotated  on  the 
colour  wheel.  After  the  extinction  of  flicker,  a  central 
white  disc  is  seen  surrounded  by  a  zone  of  grey,  which 
gradually  darkens  towards  the  periphery,  shading  into  a 
deep  black.  Between  the  white  centre  and  the  beginning 
of  the  grey  zone  occurs  a  band  of  greater  brightness  than 

*  that  of  the  former ;  while  between 
the  outer  edge  of  the  grey  zone 
and  the  surrounding  black  occurs 
a  band  of  deeper  blackness  than 
the  latter. 

To  such  conditions  of  smoothly 
graded  contrast  McDougall  has 
applied  the  hypothesis  of  "  drain- 
age" (page  100).  By  virtue  of 
their  central  nervous  connections, 
FIG.  2.  he  supposes  that  the  more  in- 

tensely stimulated  cerebro-retinal 

elements  drain  away  to  themselves  the  energy  from  the 
less  intensely  stimulated ;  so  that  the  final  effect,  in  a  disc 
of  the  above  pattern,  is  to  depress  the  excitation  effect 
where  the  grey  meets  the  black,  and  to  exalt  it  where  the 
grey  meets  the  white. 

This  hypothesis,  however,  appears  of  somewhat  doubtful 
validity.  It  must  needs  be  proved  that  the  two  bands  occur, 
when  such  a  graded  disc  is  prepared  objectively,  instead  of 
being  produced  subjectively  by  rotating  the  black  and  white 
disc  of  figure  2.  Moreover,  when  a  photographic  negative  is 
taken  of  the  latter  rotating  disc,  it  shows  the  same  bands  as 
the  disc  presents  to  the  eye.  The  phenomena  are  therefore 


VISUAL  SENSATIONS  103 

perhaps  connected  with  those  of  flicker,  and  in  that  case 
require  a  simpler,  physical  explanation.] 

The  Nature  of  "  Black." — This  subject  has  excited  con- 
siderable controversy.  It  must  be  remembered  that,  accord- 
ing to  the  Young-Helmholtz  theory,  black  is  experienced 
during  a  state  of  absolute  inactivity  in  the  three  primary 
colour  systems,  while,  according  to  Hering's  theory,  it  is  the 
result  of  assimilatory  change  in  the  black-white  apparatus. 
The  difference  between  these  two  views  indicates  the  turn 
which  the  controversy  has  always  taken.  Is  our  experience 
of  black  dependent  on  the  quiescence  of  the  cerebro-retinal 
apparatus,  or  is  it  a  sensation  which  is  fundamentally 
comparable  to  sensations  of  grey  or  white  ? 

We  have  seen  that  when,  in  the  absence  of  all  external 
stimulation,  the  eye  is  in  a  state  of  dark-adaptation,  an 
experience  not  of  black  but  of  the  so-called  intrinsic  light 
of  the  retina  results.  We  may  now  go  further,  and  state 
that  black  is  only  experienced  when  an  area  of  the  retina, 
previously  excited  by  white  light  but  now  unexcited  by 
external  stimuli,  is  in  a  state  other  than  that  of  dark- 
adaptation.  Thus,  when  first  we  enter  an  absolutely  dark 
room  after  quitting  daylight,  the  retina  is  as  yet  unadapted 
to  darkness;  hence  black  is  experienced.  Similarly,  when 
a  black  image  is  received  on  a  given  area  of  the  retina,  so 
long  as  the  rest  of  the  retina  is  stimulated  by  light,  that 
area  can  never  reach  a  state  of  complete  dark-adaptation ; 
hence,  again,  black  is  experienced.  A  pure  black  results,  as 
we  have  said,  if  the  surrounding  region  of  the  retina  is  being 
stimulated  by  white  light.  If  coloured  light  is  exciting  it, 
we  should  expect  the  black  to  be  tinged,  owing  to  simul- 
taneous contrast,  in  the  complementary  colour.  The  effect 
of  such  tingeing,  although  unnoticed  during  excitation,  may 
be  very  obvious  in  the  after-image  (exp.  60). 

But  if  black  is  an  experience  depending  on  the  temporary, 
local  or  general,  condition  of  the  cerebro-retinal  apparatus, 
we  are  surely  justified  in  placing  it  on  the  same  footing  as 


104  EXPERIMENTAL  PSYCHOLOGY 

those  other  visual  experiences  which,  as  the  outcome  of 
external  stimulation,  are  admitted  to  be  sensations.  Were 
our  experience  of  visual  sensations  dependent  solely  on  the 
presence  of  external  stimuli,  it  is,  of  course,  obvious  that  no 
sensation  could  arise  in  the  absence  of  a  stimulus ;  and 
that  therefore  it  would  be  impossible  to  regard  black  as  a 
sensation.  But  the  effects  of  successive  and  simultaneous 
contrast  forbid  such  a  view.  We  have  repeatedly  seen  that 
the  presence,  the  nature,  and  the  course  of  a  visual  sensation 
depend  not  merely  upon  the  outward  stimulus,  but  also 
upon  the  local  and  general  condition  of  the  entire  cerebro- 
retinal  apparatus.  It  is  in  no  way  absurd  to  regard  this 
local  or  general  condition  as  acting  in  the  form  of  an 
internal  positive  or  negative  stimulus  upon  the  cerebro- 
retinal  area  under  consideration.  We  are  thus  brought  to 
the  conclusion  that,  although  there  is  no  outward  stimula- 
tion corresponding  to  the  experience  of  black,  there  are 
cerebro-retinal  conditions  or  inward  stimulations,  upon  the 
existence  of  which  the  experience  of  black  is  dependent. 

We  seem  all  the  more  justified  in  regarding  black  as  a 
"  sensation,"  when  we  compare  the  effects  of  simultaneous 
and  successive  contrasts  obtainable  from  black,  white,  grey, 
and  from  colour  stimuli;  when  we  consider  the  behaviour 
of  black  in  binocular  mixture  and  binocular  rivalry  (page 
280) ;  and  especially,  when  we  bear  in  mind  the  striking 
changes  produced  by  mixing  a  given  colour  with  black  or 
with  white.  For  example,  the  sensation  of  brown  cannot 
be  obtained  by  reducing  the  intensity  of  a  yellowish-red 
spectral  light  or  of  the  light  reflected  from  a  yellowish- 
red  pigment.  The  result  of  such  a  reduction  is  that  the 
yellowish-red  more  and  more  nearly  approaches  black.  In 
order  that  a  brown  sensation  may  be  obtained,  the  stimulus 
effect  of  the  yellowish-red  light  must  be  "  blackened "  by 
simultaneous  (exp.  62)  or  by  successive  (exp.  55)  contrast, 
or  the  pigment  must  be  mixed  with  black. 

It  is  clear,  then,  we  cannot  accept  as  adequate  Helm- 


VISUAL  SENSATIONS  105 

holtz's  statement  that  we  call  an  object  black  which  reflects 
no  light  when  light  falls  upon  it.  Such  a  condition  is 
experimentally  unattainable.  Moreover,  an  object  can 
transmit  to  the  eye  a  relatively  considerable  amount  of 
light,  and  yet,  owing  to  contrast,  it  may  appear  black.  Nor 
does  the  total  withdrawal  of  retinal  stimuli  produce  the 
experience  of  black.  "  To  be  blind "  and  "  to  see  black  " 
are  two  very  different  things. 

[Other  Theories.  General  Criticism. — More  and  more 
strongly  the  conviction  is  forced  upon  us  that  visual 
sensations  are  of  a  far  more  complex  nature  than  Helm- 
holtz's  or  even  Bering's  simple  colour  theories  would  lead 
us  to  suppose.  Evidence  is  gradually  accumulating  that, 
before  a  sensation  reaches  its  full  development,  it  undergoes 
a  process  of  complicated  elaboration,  of  the  details  of  which, 
however,  we  are  as  yet  totally  ignorant.  This  elaboration 
doubtless  takes  place  at  different  stages,  at  different  nervous 
levels  in  the  cerebro-spinal  system,  and  it  is  quite  conceivable 
that  stimuli,  which  react  peripherally  on  separate  neural 
elements,  overlap  in  their  action  on  more  central  elements, 
and  that  the  direction  of  this  overlapping  differs  at  different 
levels.  As  psychologists,  we  are  by  now  far  removed  from 
the  crudities  of  the  earlier  physicists,  who  supposed  that  a 
wave  of  given  length  on  reaching  the  retina  immediately 
becomes  transformed  into  a  sensation  of  colour,  and  that 
psychical  effects  are  identifiable  with  their  external  physical 
causes. 

In  conformity  with  this  view,  Gr.  E.  Miiller  has  supposed 
that,  in  addition  to  four  chromatic  retinal  "  processes,"  red, 
yellow,  green,  blue,  at  the  periphery,  there  are  six  more 
central  "values,"  the  red  process  exciting  the  red,  yellow, 
and  white  values,  the  yellow  process  exciting  the  yellow, 
green,  and  white  values,  the  green  exciting  the  green,  blue, 
and  black,  and  the  blue  process  exciting  the  blue,  red,  and 
black  values.  He  supposes  that  a  yellow  stimulus  excites 
the  red  and  yellow  processes,  thereby  exciting  the  red, 


io6  EXPERIMENTAL  PSYCHOLOGY 

yellow,  green,  and  white  central  values,  of  which  the  red 
and  green  neutralise  one  another.  Miiller  endeavours  to 
account  for  the  varieties  not  only  of  colour  blindness,  but 
also  of  anomalous  colour  vision  (page  85),  by  the  absence 
of  one  or  more  of  the  retinal  processes  or  of  their  central 
values  or  of  both  processes  and  values  together. 

In  an  earlier  paper,  Miiller  had  substituted  the  concep- 
tion of  reversible  chemical  or  molecular  actions  for  Bering's 
antagonistic  processes  of  assimilation  and  dissimilation. 
He  also  posited  an  additional  cortical  or  sub-cortical  white- 
black  apparatus,  although  accepting  Bering's  threefold 
(red-green,  yellow-blue,  white-black)  apparatus  for  the  peri- 
phery. In  the  same  paper  Miiller  supposed  that  the 
central  white-black  apparatus  determined  the  character 
of  the  various  members  of  the  white-black  series  and  the 
brightness  of  colour  sensations.  According  to  this  view,  the 
more  peripheral  white-black  apparatus,  in  the  absence  of 
the  central  white-black  apparatus,  is  able  to  prod  ace  only 
black  or  white,  never  grey.  The  so-called  intrinsic  light  of 
the  retina,  Miiller  urged,  is  really  of  central  origin  due  to 
the  activity  of  the  central  apparatus.  This  hypothesis  is 
supported  by  the  fact  that  even  in  optic  atrophy  and  after 
extirpation  of  the  eye,  the  so-called  intrinsic  light  remains. 

Attempts  have  been  made  to  hold  a  position  inter- 
mediate between  that  of  Helmholtz  and  Bering.  Bonders 
supposed  that  a  trichromic  (red,  green,  and  violet)  system 
existed  at  the  periphery  of  the  cerebro-retinal  apparatus, 
while  a  fourfold,  red,  yellow,  green,  and  blue,  process 
occurred  at  the  centre.  Like  Miiller,  Bonders  assumed 
that  a  single  peripheral  process  might  act  on  more  than  one 
central  process. 

Similarly  von  Kries,  while  advocating  a  trichromic  pro- 
cess, suggests  that  at  the  periphery  of  the  retina  a  four- 
colour  process  is  also  involved. 

We  have  already  seen,  in  the  case  of  temperature  sen- 
sations, that  a  theory,  which  fairly  represented  the  facts  of 


VISUAL  SENSATIONS  107 

contrast  and  adaptation,  became  difficult  of  acceptance  upon 
the  discovery  of  a  differentiation  of  end  organs  at  the  peri- 
phery. Contrast,  however,  may  occur  even  when,  as  in 
taste,  the  sensations  have  apparently  independent  structural 
bases  at  the  periphery.  Bering's  theory  of  contrast  does 
not,  therefore,  preclude  the  elaboration  of  sensations  from 
peripheral  elements,  which  are  different  from  and  more 
primitive  than  those  advocated  in  his  colour  theory.  Nay, 
nothing  is  more  certain  than  that,  in  addition  to  the  peri- 
pheral processes  of  adaptation  and  fatigue,  central  processes 
of  inhibition  and  augmentation  occur.  But  at  present  we 
are  powerless  to  separate  the  one  from  the  other ;  we  can 
only  speak  of  changes  in  one  vast  unravelled  complex, — the 
cerebro-retinal  apparatus.] 


BIBLIOGRAPHY. 

In  addition  to  the  works  mentioned  in  the  last  chapter — F.  G.  Bonders, 
"Uber  Farbensysteme,"  Arch.  f.  Ophthalm.,  1881,  xxvii.  155,  255;  1884, 
xxx.  15.  F.  Hillebrand,  "Uber  d.  spezifische  Helligkeit  d.  Farben," 
SUzunysber.  d.  Wicn.  Akad.  d.  Wisscnsch.,  1889,  xcviii.  Abt.  3,  70.  G.  E. 
Miiller,  "  Zur  Psychophysik  d.  Gesichtsempfindungen,"  Ztsch.  f.  Psychol. 
u.  Physiol.  d.  Sinnesorgane,  x.,  1896,  i.  321  ;  xiv.,  189-,  i.  161  ;  "Die 
Theorie.  d.  Gegenfarben  u.  d.  Farbenblindheit,"  Bericht  uber  d.  I.  Kongress 
f.  exp.  Psychol.,  Leipzig,  1904,  6.  W.  M'Dougall,  "Some  New  Observa- 
tions in  Support  of  Thomas  Young's  Theory  of  Light  and  Colour  Vision," 
Mind,  1901  (N.S.),  x.  52,  210,  347;  "Intensification  of  Visual  Sensation 
by  Smoothly  Graded  Contrast,"  Journ.  of  Physiol.,  1903,  xxix.  p.  xix. 
J.  Ward,  "  Is  '  Black '  a  Sensation  ?  "  Brit.  Journ.  of  Psychol.,  1906,  i.  407. 


CHAPTER  VIII 
ON   GUSTATORY  AND  OLFACTORY  SENSATIONS 

Their  Close  Relation. — Taste  and  smell  are  so  similar 
and  so  closely  associated  in  experience  that  they  may  be 
conveniently  considered  in  the  same  chapter. 

The  majority  of  substances  that  are  said  to  "  taste,"  owe 
their  flavour  to  our  sense  of  smell.  If,  while  the  nostrils 
are  held,  small  pieces  of  apple  and  onion  are  alternately 
chewed,  it  is  impossible  to  distinguish  them.  But  when  the 
nostrils  are  opened  and  currents  of  air  permit  the  vapours 
of  the  apple  or  onion  to  pass  up  behind  the  soft  palate  into 
the  nose  (by  way  of  the  posterior  nares),  the  two  objects 
are  at  once  distinguished.  It  is  chiefly  by  this  path  that 
flavours  reach  the  olfactory  end  organs  on  which  they  act, 
the  nostrils  serving  for  the  examination  of  odours  before 
they  are  taken  into  the  system.  We  restrict  sensations  of 
taste  to  those  which  are  produced  by  stimulation  of  the 
taste  buds  of  the  tongue  and  of  adjacent  tissues. 

GUSTATORY  SENSATIONS. 

Simple  and  Complex  Tastes. — The  early  classifications 
included  many  tastes  (e.g.  dry,  astringent,  pungent,  oily, 
and  acrid)  which  we  now  recognise  to  result  from  ex- 
citation of  the  end  organs  not  of  taste  but  of  touch, 
temperature,  etc.,  which  are  contained  in  the  tongue. 
The  true  primary  tastes  are  sweet,  bitter,  salt,  and  sour 
(or  acid).  The  insipid,  metallic,  and  alkaline  tastes  are 

108 


GUSTATORY  AND  OLFACTORY  SENSATIONS     109 

compounded,  as  we  shall  see,  from  gustatory,  tactile,  and 
other  elements. 

Sweet  and  bitter  substances  give  purer  sensations  of 
taste  than  salt  and  sour  substances.'  Thus,  a  slightly 
burning  sensation  may  be  detected,  when  the  tongue  is 
treated  with  a  solution  of  common  salt  which  is  too  weak  to 
give  rise  to  a  salt  taste.  Extremely  weak  acid  solutions 
give  rise  to  an  astringent  sensation,  which,  with  increasing 
strength  of  the  solution,  finally  passes  over  into  pain.  The 
true  acid  or  sour  taste  may  be  separated  from  the  astringent 
effect  which  accompanies  it,  by  painting  the  tongue  several 
times  with  a  5-10  per  cent,  solution  of  cocaine.  Cocaine 
first  abolishes  the  sour  taste,  and  after  several  minutes 
begins  to  abolish  the  astringent  action  of  the  acid  solution. 
Later  the  sour  sensation  begins  to  return  while  the  astrin- 
gent effect  is  still  in  abeyance,  so  that  the  application  of  an 
acid  solution  at  a  certain  stage  during  recovery  enables  the 
true  taste  character  of  sour  to  be  differentiated. 

Sweet  and  bitter  tastes,  although  much  freer,  are  by  no 
means  entirely  divorced  from  tactile  or  similar  concomi- 
tants. Weak  solutions  of  bitter  substances  give  a  "  fatty  " 
sensation,  while  sweet  solutions  when  too  weak  to  give  a 
taste  are  smooth  and  soft,  and  when  very  intense  they  prick 
or  burn.  These  concomitant  sensations  of  touch,  tempera- 
ture, etc.,  may  also  be  studied  by  applying  the  solution  to 
tasteless  regions  of  the  mouth,  e.g.  to  the  hard  palate. 

By  mixing  strong  solutions  of  salt  and  sweet  substances 
in  certain  proportions,  the  alkaline  taste  may  be  nearly 
imitated.  It  has  been  suggested  that  the  metallic  taste 
is  due  to  the  simultaneous  development  of  salt  and  sour 
tastes.  The  failure  to  produce  exact  alkaline  and  metallic 
tastes  synthetically  is  in  part  due  to  the  difficulty  of  imitat- 
ing the  tactile  and  other  sensations  with  which  they  are 
bound  up. 

The  Chemistry  of  Tasting  Substances. — The  differences  in 
action  of  the  various  taste  stimuli  are  in  great  part  the 


i  io  EXPERIMENTAL  PSYCHOLOGY 

expression  of  differences  in  their  molecular  composition  and 
constitution.  Slight  changes  in  the  constitution  of  the 
molecule  are  sufficient  to  convert  many  sweet-tasting  into 
bitter-tasting  substances.  The  elements  which  enter  into 
combination  to  give  sweet  compounds  lie,  for  the  most  part, 
midway  between  extremely  positive  and  extremely  negative 
elements,  when  the  elements  are  ranged  according  to  the 
periodic  law. 

The  Region  of  Taste. — Sensibility  to  taste  is  spread  more 
widely  over  the  mouth  of  the  child  than  over  that  of  the 
adult.  The  middle  region  of  the  tongue  is  incapable  of 
producing  taste  sensations  in  the  adult,  but  in  the  child 
both  this  area  and  also  the  mucous  membrane  of  the  cheek 
are  efficient.  In  the  adult,  sensations  of  taste  cannot  be 
evoked  from  the  lips,  cheek,  guins,  and  uvula,  but  they 
can  be  obtained  from  the  soft  palate,  the  posterior  wall 
of  the  pharynx,  the  laryngeal  surface  of  the  epiglottis,  and 
even  from  the  larynx  itself. 

The  function  of  the  taste  buds  of  the  upper  surface  of 
the  soft  palate  and  of  the  larynx  is  altogether  obscure.  It 
has  been  suggested  that  they  are  the  source  of  the  sensations 
of  certain  so-called  "  olfactory  tastes,"  e.g.  the  sweetness  of 
chloroform,  the  frequent  bitterness  of  ether  vapour,  when 
taken  in  by  the  nostrils.  They  may,  on  the  other  hand,  be 
the  onto-  and  phylo-genetic  remains  of  the  wider  distribu- 
tion of  taste  buds  which  obtains  in  childhood  and  in  lower 
vertebrates.  Possibly  they  serve  to  reinforce  the  reflexes 
that  protect  the  laryngeal  cavity  from  contact  with  food. 

It  is  generally  believed  that  taste  sensations  can  be 
obtained  only  by  stimulation  of  taste  buds.  In  the  tongue 
the  buds  lie  in  the  circumvallate  and  fungiform  papillae, 
and  it  is  only  in  these  situations  that  stimulation  of  the 
tongue  produces  sensations  of  taste.  Some  of  the  fungiform 
papillae  seem  incapable  of  yielding  any  taste  sensations. 
Other  papilla  are  sensitive  to  two  or  three  of  the  four  classes 
of  taste  stimuli,  while  a  few  may  be  found  which  only 


GUSTATORY  AND  OLFACTORY  SENSATIONS     1 1 1 

respond  to  a  single  class  of  stimulus,  e.g.  to  sugar,  salt, 
or  tartaric  acid;  apparently  no  papillae  are  ever  sensitive 
solely  to  bitter  stimuli  (exp.  75). 

The  Action  of  Drugs. — The  differentiation  in  function 
of  the  end  organs  of  taste  is  shown  not  only  by  punctiform 
exploration,  but  also  by  the  specific  action  of  certain  sub- 
stances, notably  cocaine  and  gymnenic  acid  (exps.  76,  77). 
Both  act  deleteriously,  chiefly  on  the  end  organs  for  bitter 
and  sweet  tastes ;  cocaine  abolishing  especially  the  bitter 
taste,  and  gymnenic  acid  abolishing  especially  the  sweet, 
leaving  the  other  two  tastes  almost  or  quite  unaffected. 

Compensation,  Rivalry,  and  Contrast.  —  While  strong 
solutions  of  sweet  and  salt  substances  simultaneously 
applied  to  the  same  area  of  the  tongue,  give  rise  to  an 
alkaline  taste,  with  weak  solutions  of  these  substances  the 
corresponding  sensations  tend  to  be  neutralised.  Unless  a 
very  small  part  of  the  tongue  is  tested,  it  is  apparently 
impossible  to  arrive  at  an  absolutely  indifferent  stage. 
Generally,  when  the  compensation  between  the  two  antago- 
nistic tastes  is  complete,  an  insipid  taste  results.  Some 
observers,  however,  claim  to  be  able  to  reach  an  absolute 
zero.  The  compensation  of  tastes, — the  partial  abolition  of 
sourness  in  wine  or  of  bitterness  in  coffee  by  sugar, — is 
familiar  to  us  all.  The  best  examples  of  compensation 
appear  to  occur  between  sweet  and  salt,  the  worst  between 
sweet  and  sour  tastes. 

Rivalry  between  two  simultaneously  present  tastes  may 
occur ;  but  usually,  where  compensation  is  absent,  a  qualita- 
tively new  experience  arises  in  which  analysis  may  detect 
the  component  elements  (exp.  78). 

Contrast  effects  may  be  obtained  between  any  two  of 
the  four  taste  sensations,  save  bitter.  Some  individuals, 
especially  those  whose  sensitivity  to  tastes  is  obtuse,  fail  to 
obtain  them.  The  contrast  may  be  so  contrived  as  to  be 
either  simultaneous  or  successive.  The  easiest  obtainable 
contrast  sensation  is  usually  sweet ;  it  may  be  evoked  more 


112  EXPERIMENTAL  PSYCHOLOGY 

readily  with  an  inducing  salt  than  with  an  inducing  sour 
stimulus.  Thus,  the  (simultaneous  or  successive)  contrast 
effect  of  salt  is  to  make  distilled  water  taste  sweet,  while 
solutions  of  sugar,  previously  too  weak  to  produce  a 
sensation,  are  now  tasted  as  sweet  (exp.  79). 

We  are  wholly  ignorant  of  the  physiological  and  psycho- 
logical nature  of  compensation,  rivalry,  and  contrast  in 
taste.  Many  writers  have  attributed  them  to  central 
rather  than  to  peripheral  factors.  Indeed,  so  long  as  we 
suppose  that  the  four  primary  tastes  are  dependent  oiifour 
distinct  kinds  of  end  organs,  it  is  at  first  sight  difficult  to 
refer  compensation  and  contrast  to  peripheral  conditions. 
The  same  difficulty  confronted  us  in  dealing  with  visual 
sensations  (page  107). 

On  the  other  hand,  we  must  bear  in  mind  that  we  are 
still  wholly  ignorant  of  the  sub-cortical  connections  which 
the  peripheral  nerve  fibres,  subserving  the  development  of 
taste  sensations,  may  form  with  one  another.  Further,  we 
cannot  reject  the  alternative  view  that  the  four  primary 
tastes  are  the  result  of  four  different  modes  of  reaction  in 
one  and  the  same  end  organ,  some  end  organs  being  capable 
of  reacting  in  all  four,  others  in  less  than  four  modes;  a 
view  which  gains  some  support  from  the  fact  that  no  cases 
are  on  record  of  individuals  in  whom  only  one  taste  is 
wanting  and  the  others  are  normal. 

OLFACTORY  SENSATIONS. 

The  Conditions  of  Smell. — Odours  are  emitted  owing  to 
the  vaporisation  of  substance,  the  odoriferous  vapour  dis- 
seminating by  diffusion  from  its  source  to  the  nose.  The 
olfactory  epithelium  in  man  occupies  a  very  small  area  of 
the  nasal  mucous  membrane,  and  is  normally  bathed  in 
fluid.  In  unhealthy  conditions  of  the  nose  which  involve 
increase  or  decrease  of  the  surrounding  mucus,  the  sensi- 
tivity of  the  olfactory  epithelium  is  seriously  affected. 


GUSTATORY  AND  OLFACTORY  SENSATIONS     1 1 3 

Experimental  evidence,  although  somewhat  conflicting, 
favours  on  the  whole  the  view  that  odoriferous  substances 
must  reach  the  region  of  the  olfactory  membrane  in  gaseous 
form  in  order  to  produce  their  smell.  We  can  but  dimly 
conjecture  how  these  gaseous  particles  stimulate  the  hair-like 
processes  of  the  sensory  end  organs.  It  has  been  suggested 
that  the  olfactory  stimulus  consists  in  intramolecular  vibra- 
tions, yielding  ethereal  waves  of  such  extreme  shortness, 
that  they  are  lost  even  in  the  thinnest  layers  of  air,  and  that 
hence  it  is  essential  for  the  odorous  particles  to  come  into 
close  contact  with  the  epithelium.  According  to  this  view, 
the  nature  of  the  olfactory  sensation  is  determined  by  the 
character  of  the  intramolecular  vibrations,  which  is  in  turn 
dependent  on  the  nature  and  mode  of  grouping  of  the  atoms 
within  the  odorous  molecule. 

To  a  limited  extent  it  is  certainly  true  that  substances 
which  are  chemically  related  emit  similar  odours.  The 
elements  chlorine,  bromine,  and  iodine,  and  the  compounds 
of  sulphur,  selenium,  and  tellurium,  are  examples  of  this 
broad  principle.  Practically  all  the  odorous  elements 
belong  to  the  fifth,  sixth,  or  seventh  group  in  the  periodic 
system  of  classification.  In  addition  to  these  elements,  the 
organic  compounds  form  an  important  group  of  odorous 
bodies.  A  study  of  the  changing  nature  and  intensity  of 
the  odour  of  members  of  the  series  of  fatty  acids  or  of  fatty 
alcohols,  affords  instances  of  the  relation  between  odour  and 
chemical  constitution.  On  the  other  hand,  it  is  not  difficult 
to  find  substances  of  widely  different  constitution  which 
are  strikingly  similar  in  smell. 

Classification  of  Smells. — In  the  other  senses  which  we 
have  studied,  we  have  endeavoured  first  to  determine  the 
number  of  elementary  sensations  by  introspection,  and  then 
to  find  out  how  far  experiment  and  observation  in  health 
and  disease  justify  us  in  recognising  these  sensations  as 
primary,  and  in  regarding  each  of  them  as  the  resulting 
activity  of  a  different  specific  apparatus.  It  is,  of  course, 
8 


114  EXPERIMENTAL  PSYCHOLOGY 

difficult  to  construct  on  an  introspective  basis  a  system  of 
classification  of  smells  which  shall  be  generally  acceptable ; 
but  the  following  scheme,  suggested  by  Zwaardemaker, 
imperfect  as  it  is,  may  serve  as  an  example  : — 

I.  Ethereal    smells. — (a)    Fruit    ethers,      (b)    Beeswax. 

(c)  Ethers,  aldehydes,  ketones  (lower  members  of 

homologous  series). 
II.  Aromatic   smells. — (a)  Camphor.      (b)  Spicy    smells. 

(c)  Anise   and    lavender.      (d)  Lemon    and    rose. 

(e)  Almond  smells. 

III.  Balsamic    smells. — (a)  Jasmine,   ylang-ylang,   orange 

blossom,     (b)  Lily-like  smells,     (c)  Vanilla  smells. 

IV.  Amber-musk  smells. — (a)  Amber,     (b)  Musk  smells. 
V.  Allyl-cacodyl  smells. — (a)  Sulphuretted  hydrogen,  asa- 

foatida  and  like  smells,   (b)  Fishy  smells,    (c)  Halo- 
gen smells. 
VI.  Burning  smells. — (a)  Toast,   tobacco   smoke,  creosol, 

etc.     (b)  Benzol,  phenol,  etc. 
VII:  Caprylic    smells. — (a)  Caproic    acid,    cheese,    sweat. 

(b)  Cat's  urine,  sexual  odours. 
VIII.  Repulsive  smells. — (a)  Narcotic  smells,     (b)  The  smell 

of  bugs,  ozaena. 
IX.  Nauseating  smells. — (a)  Putrefying  bodies,    (b)  Fseces, 

scatol. 

Anosmia. — An  unsatisfactory  attempt  has  been  made  to 
allot  to  the  substances  in  these  different  classes  certain 
"  odoriferous "  atom  groups,  to  which  their  smell  may 
hypothetical^  be  ascribed.  But  the  system  of  classification 
receives  some  limited  support  from  observations  on  the  order 
of  recovery  of  sensibility  for  various  odours,  after  a  total 
loss  of  smell  (general  anosmia)  has  been  temporarily  pro- 
duced by  artificial  means.  It  is  said  that  the  odour  of 
bodies  belonging  to  class  VI.  returns  first ;  then  follows 
recovery  of  sensibility  towards  bodies  belonging  to  class  VII. 
Classes  V.  and  IX.  return  next,  then  classes  L,  II.,  and  III,, 
and  lastly  classes  IV.  and  VIII. 


GUSTATORY  AND  OLFACTORY  SENSATIONS     1 1 5 

The  hallucinations  and  the  defects  of  smell,  when  more 
carefully  studied,  may  be  expected  to  throw  further  light  on 
the  primary  olfactory  sensations.  Many  people  are  unable 
to  smell  prussic  acid.  Cases  are  on  record  of  individuals 
who,  while  possessing  in  other  respects  normal  olfactory 
sensations,  are  more  or  less  completely  incapable  of  smelling 
one  or  other  of  the  following  odours  : — mignonette,  vanilla, 
violets,  frisia,  or  benzoin.  In  the  study  of  defects  of  smell, 
it  must  not  be  forgotten  that  many  stimuli  (e.g.  ammonia) 
excite  not  only  the  special  olfactory  epithelium,  but  also  the 
general  respiratory  epithelium  of  the  nose,  giving  rise  to 
tactile,  pricking,  tingling,  or  painful  sensations  (exp.  81). 
We  have  already  noted  (page  110)  that  certain  olfactory 
stimuli  also  yield  sensations  of  taste. 

Fatigue. — The  olfactory  epithelium  is  easily  fatigued.  Yet 
the  effects  of  exhaustion  produced  by  a  single  odour  do  not 
affect  the  sensitivity  of  the  nose  to  all  kinds  of  odours  (exp. 
82).  Here  again  experiment  appears  to  point  to  a  differ- 
entiation of  function  of  the  sensory  apparatus,  certain  odours 
acting  on  certain  end  organs,  other  odours  acting  on  others. 

The  same  conclusion  is  indicated  by  the  gradual  changes 
in  character  which  many  odours  undergo  while  they  are 
being  continuously  smelled  (exp.  83). 

Compensation  and  Rivalry. — The  use  of  perfumes  in 
disguising  objectionable  smells  shows  that  one  odour  can  be 
concealed  by  another.  This  is  quite  independent  of  any 
chemical  interaction  between  such  antagonistic  odours. 
Under  favourable  conditions,  the  two  odours  may  be  so 
adjusted  that  a  state  of  complete,  or  nearly  complete,  com- 
pensation is  reached.  A  more  careful  analysis  of  this 
phenomenon  of  compensation  may  be  expected  to  lead  to  an 
improved  classification  of  olfactory  sensations.  How  far  the 
phenomenon  depends  on  central,  how  far  on  peripheral 
factors,  is  at  present  unknown ;  but  it  is  noteworthy  that 
compensation  between  certain  odours  may  be  obtained  when 
they  are  introduced  to  separate  nostrils. 


ii6  EXPERIMENTAL  PSYCHOLOGY 

Zwaardemaker  suggests  that  nine  systems  of  apparatus 
exist  corresponding  to  his  nine  classes  of  olfactory  sensa- 
tions, and  that  they  are  ranged  in  spatial  order,  those  groups 
of  apparatus  stimulated  by  the  food  smells  being  placed 
most  anteriorly,  those  which  are  stimulated  by  the  nauseat- 
ing smells  being  placed  most  posteriorly,  while  the  sexual 
odours  are  represented  midway.  He  believes  that,  if  two 
odours  belonging  to  neighbouring  classes  are  simultaneously 
presented  to  the  nose,  a  new  sensation  results,  apparently 
simple,  but  really  derived  from  the  elementary  constituents  ; 
whilst,  when  the  odours  simultaneously  presented  belong  to 
more  distant  classes,  compensation  results  if  the  intensity  of 
the  odours  be  weak,  and  rivalry  of  the  two  odours  results 
if  they  be  strong  (exp.  84).  There  are,  however,  many 
difficulties  in  the  way  of  accepting  these  views. 

BIBLIOGRAPHY. 

E.  Aronsohn,  "Zur  Physiologic  des  Geruchs,"  Arch.  f.  \_Aiicit.  u.~\ 
Physiol.,  1884,  163;  "  Experimentelle  Untersuchungen  zur  Physiol.  d. 
Geruchs,"  ibid.,  1886,  321.  J.  B.  Haycraft,  "The  Nature  of  the  Objective 
Cause  of  Sensation,"  Brain,l  887,  x.  145;  1888,  xi.  166.  H.  Oehrwall,"  Unter- 
suchungen liber  d.  Geschmacksinn,  Skand.  Arch.  f.  Physiol.,  1891,  ii.  1. 
L.  E.  Shore,  "A  Contribution  to  our  Knowledge  of  Taste  Sensation,"  Journ. 
of  Physiol.,  1892,  xiii.  191.  F.  Kiesow,  "  Uber  d.  Wirknng  d.  Cocain  u.  d. 
Gymnemasaure  auf  d.  Schleimhaut  d.  Zunge  di.  d.  Mundraums,"  Philosoph. 
Stud.,  1894,  ix.  510  ;  "Beitrage  zur  physiol.  Psychol.  d.  Geschniacksinnes," 
ibid.,  1894,  x.  329,  523;  1896,  xii.  255,  464.  H.  Zwaardemaker,  Die 
Physiologic  des  Geruchs,  1895;  "Die  Compensation  von  Geruchsempfind- 
ungen,"  Arch.  f.  \_Anat.  u.]  Physiol.,  1900,  423  ;  "Geschmack,"  in  Asher 
and  Spiro's  Ergebnisse  d.  Physiol.,  Wiesbaden,  1903,  Abt.  2,  ii.  699.  W. 
Nagel,  "  Uber  Mischgeriiche  u.  d.  Komponentengliederung  d.  Geruchsinnes," 
Ztseh.  f.  Psychol.  u.  Physiol.  d.  Sinnesorgane,  1897,  xv.  82;  "Der  Geruch- 
sinn,"  in  Nagel's  Handbuch  d.  Physiol.  d.  Menschen,  Braunschweig,  1905, 
iii.  589;  "Der  Geschmackssinn,"  ibid.,  621.  A.  Rollett,  "Beitrage  zur 
Physiol.  d.  Geruchs,  d.  Geschmacks,  u.s.w.,"  Arch.  f.  d.  ges.  Physiol., 
1899,  Ixxiv.  383.  G.  Marchand,  Le  goiU,  Paris,  1903.  W.  Sternberg, 
"Zur  Physiol.  d.  siissen  Geschmacks,"  Ztschr.  f.  Psychol.  u.  Physiol.  d. 
Sinnesorgane,  1904,  xxxv.  81  ;  Geschmack  u.  Genich,  Berlin,  1906.  C.  S. 
Myers,  "  The  Taste-names  of  Primitive  Peoples,"  Brit.  Journ.  of  Psychol., 
1904,  i.  117. 


CHAPTEE  IX 
ON  THE  SPECIFIC  ENERGY   OF  SENSATIONS 

Adequate  and  Inadequate  Stimuli. — The  "adequate" 
stimulus  to  any  sense  organ  is  the  special  kind  of  stimulus 
to  which  the  sense  organ  is  adapted  to  respond.  Light,  for 
instance,  is  the  adequate  stimulus  in  the  case  of  the  retina, 
sound  in  the  case  of  the  cochlea.  Other  kinds  of  stimuli 
to  which  a  sense  organ  may  respond,  are  spoken  of  as 
"  inadequate  "  stimuli. 

There  is  good  reason  to  believe  that  every  sense  organ, 
whether  it  be  excited  by  an  adequate  or  inadequate  stimulus, 
gives  rise  only  to  its  own  specific  sensation.  Most  observers 
agree  that  a  cold  spot  on  the  skin  may  be  excited  by  cold,  by 
pressure,  by  warmth,  or  by  a  faradic  current ;  but  in  each 
case  a  sensation  only  of  cold  is  evoked  (exp.  3).  So,  too, 
an  electric  stimulus  produces  a  sensation  of  pain,  heat,  cold, 
or  pressure,  according  as  it  is  applied  to  a  pain,  a  heat,  a 
cold,  or  a  pressure  spot ;  a  blow  on  the  eye  produces  visual, 
a  blow  on  the  ear  auditory,  sensations. 

Effective  and  Ineffective  Stimuli. — On  the  other  hand, 
sense  organs  do  not  respond  to  every  form  of  inadequate 
stimulus.  The  ear,  for  example,  is  not  stimulated  by  waves 
of  light,  nor  the  eye  by  waves  of  sound.  Stimuli  may 
thus  be  classed  as  effective  and  ineffective.  Electrical  and 
mechanical  (e.g.  blows  or  pressure)  are  the  most  generally 
effective  of  all  stimuli. 

The  effectiveness  of  electrical  stimuli  is  often  partly  due 
to  the  adventitious  production  of  adequate  stimuli.  Thus 


Ii8  EXPERIMENTAL  PSYCHOLOGY 

the  auditory  sensations,  arising  from  the  application  of  a 
current  to  the  ear,  are  in  part  the  result  of  sounds  occasioned 
by  the  contracting  muscles  of  the  middle  ear.  And  the 
gustatory  sensations,  electrically  evoked  from  the  tongue, 
are  in  part  due  to  the  tasting  substances  which  are  produced 
by  the  electrolytic  decomposition  of  salivary  and  mucous 
secretion.  But  altogether  aside  from  such  secondary  effects, 
electrical  stimuli  seem  to  be  universally  and  directly 
effective  as  inadequate  stimuli. 

Specific  Nervous  Energy.  —  Johannes  Miiller  supposed 
that  every  sensory  nerve  or  every  sensory  nerve  centre 
possesses  its  own  "  specific  nervous  energy  "  which  gives  rise, 
in  the  case  of  the  eye,  for  example,  to  visual,  in  the  case  of 
the  ear  to  auditory  sensations.  Miiller 's  theory  was  that 
any  stimulus,  so  long  as  it  is  effective,  invariably  evokes 
the  same  specific  energy  in  a  given  sensory  apparatus.  He 
allowed,  however,  that  any  sensory  apparatus  may  respond 
in  a  variety  of  ways  to  different  forms  of  adequate  stimuli. 
This  admission  serves  to  explain  the  various  colour  sensa- 
tions producible  in  the  retinal  apparatus,  and  similar 
phenomena  in  other  sense  organs. 

It  is  true  that  we  have  no  anatomical  or  physiological 
evidence  in  favour  of  the  existence  of  specific  nervous 
energies  in  different  sensory  nerves  or  in  different  sensory 
centres.  So  far  as  we  know,  the  nervous  impulses  passing 
to  the  brain  along  a  gustatory  nerve  do  not  differ  in  any 
way  from  those  passing,  say,  along  an  auditory  nerve.  Nor 
is  it  clear  how  the  cortical  centres  for  taste  and  for  hearing 
can  owe  their  differentiation  of  function  to  differences  in 
their  structure,  connections,  or  chemical  composition.  On 
the  other  hand,  if  we  cling  to  the  hypothesis  of  psycho- 
physical — or  rather  of  psycho-physiological  (page  9) — 
parallelism,  we  cannot  but  believe  that  ultimately  a  satis- 
factory physiological  basis  will  be  found  for  these  differences 
in  sensation,  produced  by  the  stimulation  of  different  kinds 
of  end  apparatus. 


SPECIFIC  ENERGY  OF  SENSATIONS         119 

The  importance  of  the  peripheral  sense  organs  in  initiat- 
ing the  various  specific  forms  of  sensation  can  hardly  be 
overrated.  If  the  end  organs  have  been  congenitally 
useless  and  remain  so,  no  experience  to  which  they  would 
normally  give  rise  is  ever  possible.  Individuals  who  are 
born  blind  (or  indeed  who  lose  their  eyesight  within  the 
first  few  years  of  infancy)  have  no  visual  sensations  and  no 
visual  images  when  awake  or  when  dreaming  in  later  life. 

But  when  once  the  corresponding  cortical  centres  have 
been  educated,  their  specific  activity  may  be  subsequently 
evoked  by  stimulation  either  of  the  centres  themselves  or 
of  their  appropriate  sensory  nerves.  Stimulation  of  the 
chorda  tympani  nerve  in  the  ear  gives  rise  to  sensations  of 
taste :  stimulation  of  the  optic  nerve  after  excision  of  the 
eye, — in  certain  circumstances  at  least, — produces  a  sensa- 
tion of  light.  Even  when  the  sensory  nerve  is  completely 
degenerated,  as  in  atrophy  of  the  optic  nerve,  the  cortical 
centres  may  still  be  excited,  and  hallucinations  of  vision 
may  occur. 

Quality  and  Modality.  —  The  result  of  an  appeal  to 
experience  is  to  establish  two  kinds  of  differences  between 
sensations,  —  differences  of  "  quality "  and  differences  of 
"modality."  Two  sensations  are  said  to  be  qualitatively 
different  from  one  another,  when  it  is  possible  to  pass 
insensibly  from  the  one  to  the  other  by  means  of  a  series 
of  insensibly  changing  intervening  sensations.  They  are  said 
to  be  modally  different  when  such  a  transition  is  impossible. 

Thus,  any  two  visual  sensations  differ  in  quality  as 
regards  colour  or  colourlessness,  inasmuch  as  we  can  pass 
from  any  one  sensation  to  the  other  by  gradual  imperceptible 
changes.  A  gustatory  sensation,  on  the  other  hand,  differs 
from  a  visual  sensation  in  modality ;  no  gradual  transition 
is  possible  between  the  sensations  of  sweetness  and  greyness. 
So,  too,  we  cannot  pass  from  an  olfactory  to  an  auditory 
sensation,  or  from  a  motor  to  a  thermal  sensation.  Such 
sensations  differ  in  modality. 


120  EXPERIMENTAL  PSYCHOLOGY 

Primary  Sensations.  —  We  can  hardly,  however,  be 
content  to  ascribe  specificity  of  energy  to  modally  different 
sensations  and  to  deny  it  to  qualitatively  different  sensa- 
tions. We  have  already  travelled  far  beyond  Miiller's 
hypothesis  in  which  a  single  specific  nervous  energy  is 
provided  for  each  sense  organ ;  we  have  sought  for  various 
specific  nervous  energies  within  the  individual  sense  organs. 

In  the  case  of  vision,  we  have  collected  evidence  which 
indicates  that  the  numerous,  introspectively  distinct,  colour 
and  colourless  sensations  are  the  functional  result  of  a 
relatively  very  small  number  of  specific  or  "primary" 
sensations.  The  same  result  is  indicated  in  the  case  of 
olfactory  gustatory  and  cutaneous  sensations. 

It  is  impossible  at  present  to  determine  the  number  of 
primary  sensations  in  respect  of  hearing.  Many  writers 
have  assumed  that  there  are  as  many  distinct  end  organs  in 
the  cochlea  as  there  are  distinguishable  sensations  of  tone, 
but  this  assumption  is  demonstrably  wrong.  For  let  us 
suppose  that  the  ear  is  just  able  to  distinguish  between  an 
auditory  stimulus  of  999*5  vibrations  and  one  of  1000,  and 
between  a  stimulus  of  1000  and  one  of  1000-5  vibrations 
per  second.  It  has  been  said  that  this  implies  that  a 
specific  end  organ  exists  for  each  of  these  three  stimuli,  and 
that  the  reason  why  we  cannot  distinguish  between,  say, 
999*8  and  1000  vibrations  is  because  they  both  act  on  the 
same  end  organ.  For  a  like  reason  stimuli  of  1000  and 
1000*3  vibrations  per  second  are  indistinguishable.  But 
were  this  the  true  explanation,  it  would  at  once  follow  that 
the  two  stimuli  of  999*8  and  1000*3  vibrations  should  be 
indistinguishable,  whereas  in  fact  they  can  be  distinguished 
from  one  another  as  easily  as  stimuli  of  1000  and  1000*5 
vibrations.  We  conclude,  then,  that  the  number  of  just 
distinguishable  sensations  affords  no  clue  to  the  number  of 
primary  sensations  or  of  specifically  different  end  organs. 

Moreover,  even  if  there  be  grounds  for  believing  that 
within  a  given  sense  organ  a  few  primary  sensations, 


SPECIFIC  ENERGY  OF  SENSATIONS         121 

variously  combined,  are  responsible  for  all  the  sensations  to 
which  it  gives  rise,  we  are  nevertheless  not  warranted  in 
concluding  forthwith  that  that  sense  organ  contains  corre- 
sponding, structurally  distinct,  end  organs.  In  the  case  of 
the  cerebro-retinal  apparatus,  for  example,  no  satisfactory 
evidence  has  been  adduced  in  favour  of  attributing  different 
specific  colour  functions  to  different  cones  of  the  fovea,  to 
different  optic  fibres,  or  to  different  parts  of  the  visual 
cortex. 

In  such  cases,  two  alternative  assumptions  are  possible. 
In  the  first  place,  we  may  assume  that  one  and  the  same 
"  specific  energy "  (to  use  Miiller's  terminology)  may  mani- 
fest itself  in  two,  three  or  more  abruptly  or  gradually 
different  ways.  This  may  be  the  case  in  vision  and  in  taste. 
It  was  also  generally  thought  to  occur  in  pain,  many 
writers  still  holding  that  pain  may  arise  from  the  over- 
stimulation  of  any  sensory  end  organ. 

In  the  second  place,  it  is  conceivable  that  the  primary 
sensations  lie  beyond  our  ken,  for  the  very  reason  that  they 
are  never  separately  experienced.  If  from  birth  onwards, 
our  simplest  obtainable  sensations  arise  from  the  excitation 
of  more  than  a  single  primitive  sensory  apparatus,  it  is  clear 
that  no  normal  individual  can  ever  obtain  the  experience 
which  the  isolated  activity  of  such  a  primitive  apparatus 
produces. 

From  two  different  aspects  this  latter  conception  may  be 
said  to  hold  for  tonal  sensations.  In  the  first  place,  we  have 
already  pointed  out  (pages  57,  58)  that  the  cochlea  probably 
never  receives  a  pure  tonal  stimulus  which  is  absolutely 
devoid  of  overtones.  It  is  thus  impossible  for  us  ever 
to  have  the  pure  tonal  sensation,  which,  according  to 
Helinholtz's  theory,  would  arise  from  the  stimulation  of  a 
given  basilar  fibre  in  the  cochlea.  In  the  second  place,  we 
have  also  shown  reasons  (pages  52,  53)  for  believing  that,  if 
Helmholtz's  theory  is  to  be  accepted,  the  pitch  of  a  sensa- 
tion must  depend  not  merely  on  the  particular  basilar  fibre 


122  EXPERIMEiNTAL  PSYCHOLOGY 

stimulated,  but  on  the  position  of  the  most  intensely  stimu- 
lated basilar  fibre.  Thus  we  can  never  experience  the 
result  of  stimulating  a  single  basilar  fibre,  and  our  tonal 
sensations  are  the  result  of  a  fusion  between  various 
primordial  elements  of  which  we  must  always  remain 
ignorant.  A  somewhat  similar  condition  is  conceivably 
true  of  olfactory  sensations. 

The  fusions  which  we  can  experimentally  bring  about,  by 
simultaneously  presenting  different  sensory  stimuli,  show 
how  different  are  the  psycho-physiological  factors  which 
determine  our  experiences  in  the  different  senses.  It  is 
sufficient  to  mention  the  very  different  kinds  and  degrees 
of  fusion  obtained  by  mixing  two  or  more  similar  or 
antagonistic  colour  or  colourless  stimuli,  by  mixing  two  or 
more  tones  of  similar  and  of  dissimilar  pitch,  or  by  mixing 
gustatory  or  olfactory  solutions. 


BIBLIOGRAPHY. 

J.  Miiller,  Elements  of  Physiology  (Eng.  trans.),  London,  1842,  ii.  1059. 
R.  H.  Lotze,  Medicinische  Psychologic,  Leipzig,  1852,  182.  H.  L.  F.  von 
Helmholtz,  Die  Thatsachen  in  d.  Wahrnehmung,  Berlin,  1879,  8  ;  Handbuch 
d.  Physiol.  Optik,  2te  Aufl.,  1894,  584.  E.  Hering,  "Ueber  d.  specifischen 
Energieen  d.  Nervensystems, "  Lotos,  1884  (N.F.),  v.  113.  C.  Stumpf, 
Tonpsychologie,  Leipzig,  1890,  ii.  106.  H.  Oehrwall,  "Untersuch.  iiber  d. 
Geschmacksinn,"  Skand.  Arch.  f.  Physiol.,  1891,  ii.  1  ;  "Ueber  d.  Modali- 
tats-  u.  Qualitatsbegriffe  in  d.  Sinnesphysiol."  ibid.,  1901,  xi.  245.  F. 
Kiesow,  ' '  Schmeckversuche  an  einzelnen  Papillen,"  Philosoph.  Stud.,  1898, 
xiv.  591.  A.  Goldscheider,  "Die  Lehre  von  d.  specifischen  Energieen," 
Gesammelte  Abhandlungcn,  Leipzig,  1898,  i.  1.  W.  Wundt,  Grundziige  d. 
physiol.  Psychol.,  Leipzig,  5te  Aufl.,  1902-3,  i.  330,  337;  ii.  15,  52,  123, 
132;  iii.  711. 


CHAPTEE  X 
ON  STATISTICAL  METHODS1 

Their  Importance.  —  It  is  incompatible  with  scientific 
method  to  draw  conclusions  from  the  result  of  a  single 
observation.  The  reliability  of  a  single  observation  must 
be  tested  by  several  repetitions  of  the  experiment  under 
conditions  as  precisely  constant  as  possible.  In  such  a 
series  of  repeated  experiments  we  can  rarely  obtain  iden- 
tical results.  Where  the  results  appear  to  be  identical, 
more  refined  methods  of  measurement  will  surely  show 
minute  differences. 

This  discrepancy  between  individual  results,  due  to  the 
variation  of  uncontrollable  circumstances,  increases  with 
the  complexity  of  the  conditions  of  experiment.  In  psy- 
chology it  is  only  natural  that  stricter  attention  should 
be  paid  to  such  discrepancies  than  in  other  branches  of 
science,  for  nowhere  are  the  experimental  conditions  so 
complicated  as  in  the  investigation  of  mental  phenomena. 

Indeed,  it  is  important  at  the  outset  to  recognise  that 
average  results  are  of  relatively  small  value  in  psychology. 
Whereas  physical  science  treats  errors  of  observation  as 
accidental,  and  endeavours,  so  far  as  possible,  to  eliminate 
them,  in  psychological  science  the  causation  of  these  errors 
is  the  prime  object  of  investigation.  From  the  psychological 
standpoint  an  average  is  often  a  blurred  result,  the  chief 
value  of  which  is  to  draw  attention  to  the  individual 
variations,  and  to  the  conditions  producing  them. 

1  See  footnote  to  Chapter  III. 
123 


124  EXPERIMENTAL  PSYCHOLOGY 

The  Mode. — The  "  mode  "  is  the  value  of  that  measure- 
ment which  occurs  more  frequently  than  any  other  in  a 
large  series  of  observations. 

The  Mean. — The  "  mean  "  or  "  average  " l  is  obtained  by 
dividing  the  sum  of  the  values  by  the  number  of  the  indi- 
vidual observations.  The  reliability  of  the  mean  depends, 
in  the  first  place,  on  the  number  of  the  observations  which 
have  been  made,  and,  in  the  second  place,  on  the  variability 
of  the  individual  values,  that  is,  on  the  extent  to  which 
they  each  diverge  from  the  mean.  No  reliance  can  be 
placed  on  a  published  mean  unless  it  is  accompanied  by 
information  on  these  two  points.  The  variability  of  the 
individual  values  may  be  best  expressed  either  by  the  "  mean 
variation  "  or  by  the  "  standard  deviation." 

The  Mean  Variation. — The  "  mean  variation,"  or  the 
"  mean  variable  error,"  commonly  denoted  by  the  letters 
m.  v.,  is  the  mean  of  the  individual  variations  from  the  mean, 
regardless  of  algebraic  sign  (exp.  85). 

The  Standard  Deviation. — The  "  standard  deviation  "  or 
the  "  mean  square  error," 2  denoted  by  the  letter  cr,  is  the 
square  root  of  the  mean  of  the  squares  of  the  individual 
variations  from  the  mean  (exp.  85). 

[The  Coefficient  of  Variation. — The  deviations  of  indi- 
vidual data  from  their  mean  depend  for  their  absolute  size, 
not  only  on  the  uniformity  of  the  experimental  conditions, 
but  on  the  absolute  size  of  the  mean.  Thus  a  mean  of 
200  which  has  a  standard  deviation  of  20  need  riot  be  less 
reliable  than  a  mean  of  100  which  has  a  standard  deviation 
of  10.  Accordingly,  a  measure  of  relative  dispersion  is 
sometimes  employed,  the  standard  deviation  being  expressed 
as  a  percentage  of  the  mean.  This  measure  has  been 
termed  the  "  coefficient  of  variation  "  (exp.  85).] 

1  These  terms  are  used  throughout  this  book  to  refer  to  the  arithmetic 
mean,  but  sometimes  they  are  given  a  wider  signification. 

2  "Root  mean  square  error"  is  a  more  accurate,  though  more  cumbrous, 
expression. 


STATISTICAL  METHODS  125 

[Significant  Differences. — It  is  obvious  that  the  mean  of 
so  small  a  series  of  observations  as  that  with  which  we 
have  to  content  ourselves  in  any  scientific  experiment,  is 
only  an  approximation  to  the  value  obtained  from  a 
larger  series.  Different  values  of  the  mean  would  result 
from  successive  series  of  observations.  In  other  words, 
the  mean,  experimentally  obtained,  is  only  the  mean 
of  a  single  sample  of  observations,  different  samples 
of  which  are  certain  to  give  somewhat  different  means. 
This  fact  becomes  of  enormous  importance,  when  one  set 
of  experimental  conditions  produces  a  mean  result,  which 
differs  from  that  obtained  under  a  purposely  different 
set  of  experimental  conditions.  The  question  then  arises, 
Is  this  difference  between  the  means  significant  or  is  it 
accidental, — that  is  to  say,  does  it  really  express  the  effect 
of  the  intentionally  altered  experimental  conditions,  or  may 
it  not  after  all  be  due  to  the  chances  of  sampling  above 
referred  to  ? 

To  settle  this  question,  we  have  to  consider  the  probable 
distribution  of  such  chance  variations  of  the  mean,  say,  in  a 
thousand  samples  of  observations,  each  sample  consisting  of, 
say,  two  hundred  measurements,  and  obtained  under  condi- 
tions as  constant  as  possible.  Clearly,  the  values  of  the 
thousand  different  means  will  range  about  a  single  mean  of 
the  possible  means.  The  greater  number  of  values  will 
occur  at  or  near  this  point,  and  others  will  occur  in  diminish- 
ing number  as  we  recede  from  it.  If  we  assume  (and  in  the 
absence  of  other  information  we  have  no  alternative  than  to 
assume)  that  the  single,  experimentally  obtained,  mean  is 
identical  with  the  value  of  the  mean  of  the  thousand 
possible  means,  we  are  able  to  study  the  mode  of  distribution 
of  the  other  means  on  either  side  of  it,  by  having  recourse 
to  the  properties  of  the  "  normal "  or  "  probability  "  distribu- 
tion curve  of  Gauss's  law  of  error.] 

[The  Normal  Curve.  —  In  order  to  understand  this 
application  of  the  normal  curve,  let  us  suppose  that  a  con- 


126  EXPERIMENTAL  PSYCHOLOGY 

siderable  number  of  measurements  be  made  of  a  complex 
variable  character,  e.g.  by  determining  the  successive  results 
of  throwing  heaps  of  coins  or  by  determining  the  stature 
of  unselected  individuals  within  a  given  community.  If 
now  such  a  series  of  observations  (or  "  variates,"  as  they  are 
often  called)  be  plotted  out  in  a  frequency  curve — the 
distances  along  the  abscissa  corresponding  to  the  different 
values  of  the  character,  the  height  of  the  ordinates  erected 
at  these  distances  corresponding  to  the  number  of  individual 
variates  that  have  the  corresponding  character, — the  curve 
will  be  found  to  approximate  in  form  to  that  of  the  normal 

or  probability  curve  (fig.  3), 
which  has  well-known  mathe- 
matical properties. 

This  normal  curve  is  some- 
times called  the  binomial  curve, 
because  the  values  of  the  ordi- 
nates follow  the  coefficients  of 
the  binomial  expansion  of  an 
expression  (x+y)m,  in  which  the 
two  terms  are  equal  to  one 
^  another.  Thus,  if  m  =  10,  the 

FlG  3  following    ordinates    will   mark 

the  course  of  the  curve : — 

10,(^=)45.(10iX29X38  =  )l20,210. 210,120,45,10. 

That  is  to  say,  the  ordinates,  erected  at  certain  equidistant 
points  of  the  abscissa  to  meet  the  curve,  will  have  those 
values. 

The  mode  is  at  the  point  A,  on  the  abscissa,  at  which 
the  maximal  ordinate  occurs. 

The  standard  deviation  marks  the  point  on  the  abscissa 
at  the  point  of  inflexion  of  the  normal  curve,  i.e.  where  the 
form  of  the  latter  changes  from  concave  to  convex. 

A  knowledge  of  the  mathematical  properties  of  the 
normal  curve  enables  us  to  calculate  the  value  of  the 


STATISTICAL  METHODS  127 

ordinate  (i.e.  the  frequency  of  occurrence)  at  any  point 
on  the  abscissa  (i.e.  in  the  case  of  any  magnitude  of 
character),  provided  that  the  values  of  A,  the  mean,  <r,  the 
standard  deviation,  and  n,  the  number  of  variates,  are 
known. 

While  cr,  the  standard  deviation,  expresses,  as  we  have 
already  seen,  the  degree  of  scatter  of  individual  variates  about 

(j 

the  mean,  —  =  gives  the  standard  deviation  or  scatter  of  the 
V  n 

numerous  values  of  the  mean  derived  from  many,  say  a 
thousand,  samples  like  the  present  one.  The  distribution 
curve  of  the  possible  means,  like  that  of  the  individual 
variates  within  the  present  sample,  conforms  to  the  normal 
frequency  curve.] 

[The  Probable  Error.  —  It  can  be  shown  that  50  per  cent. 
of  these  various  possible  means  lie  within  the  limiting 
values  which  are  greater  or  less  than  the  mean  of  those 

0-6745cr  . 


. 

means  by  --  ;=-  '    This  expression  ±        .  —  —  is  known  as 
J       V  n  ^  n 

the  probable  error,  E,  of  the  mean.  That  is  to  say,  there 
are  even  chances  that  a  mean  obtained  from  any  other 
sample  will  lie  within  the  limits  A  ±  E.  It  can  also  be 
shown  that  the  chances  of  any  value  of  the  mean  lying 

within  the  limits  of  A  ±  2  E  are  about      4-6    1 


A  ±  3  E  „  16 

A  ±  4  E  „  142 

A  ±  5  E  „  1341 

A  ±  6  E  „  19304 

A  ±  7  E  „  426910 


The    chances    against    any    found    value     of     the     mean 

E  E 

lying   within   the  limits   A  ±  T>'  A  ±  -g>  etc.,  can  be  also 

calculated. 

Thus  we  have  reached  a  stage  in  which  we  have  know- 


128  EXPERIMENTAL  PSYCHOLOGY 

ledge  of  the  distribution  (i.)  of  the  individual  variates  of  a 
sample  series,  and  (ii.)  of  the  individual  means  of  different 
sample  series. 

We  can  now  return  to  our  original  question  which  presup- 
posed that,  under  expressly  altered  experimental  conditions, 
we  have  obtained  two  different  series,  each  with  a  different 
mean.  We  required  to  know  when  the  difference  between 
two  means  may  be  regarded  as  significant,  and  when  it  is 
more  likely  to  be  due  to  chance  sampling. 

If  Al  and  A2  be  the  means  of  the  two  series,  it  is  clear 
that  the  distribution  of  the  different  values  of  these  means 
in  different  sample  series  can  be  determined,  when  their 
probable  errors  EAX  and  EA2  have  been  calculated.  But  we 
have  now  to  consider  the  distribution  of  different  values  of 
the  possible  differences  between  these  different  values  of  Al 
and  A2.  The  probable  error  of  the  difference  of  two  means 
can  be  shown  to  be  equal  to  the  square  root  of  the  sum 
of  the  squared  probable  errors  of  these  means,  that  is, 


When  the  difference  between  two  means  turns  out  to  be 
only  equal  to  the  probable  error  of  the  difference  between 
them,  i.e.  when  A1  —  A2  =  EAJ-AJ,  the  chances  are  only 
about  three  to  one  against  an  equal  or  greater  difference  of 
the  same  sign  occurring  in  a  case  of  pure  sampling.  In 
order  that  an  observed  difference  between  two  means  can 
be  safely  accepted  as  significant,  it  must  at  least  exceed 
four  and  a  half  times  the  value  of  the  probable  error  of  the 
difference  (exp.  86).  Thus  we  have  at  length  answered 
the  question  which  confronted  us  on  page  125.] 

The  Median.  —  Besides  the  mean  and  the  mode,  a  third 
expression,  the  "median"  (Mdn.),  is  sometimes  employed  in 
order  to  generalise  from  the  individual  data  of  a  given 
series.  The  variates  are  arranged  in  their  order  of  magni- 
tude, and  the  median  is  that  value  above  and  below  which 
the  variates  occur  in  equal  numbers  (exp.  85). 

[If   the  distribution  of   the  variates  obeys  that  of  the 


STATISTICAL  METHODS  129 

probability  curve, — and  for  a  considerable  number  of 
different  psychological  investigations,  provided  that  the 
series  be  large  enough,  this  has  been  shown  to  be  nearly 
or  absolutely  the  case, — the  median  value  is  identical  with 
the  mode  and  mean.  Some  distributions,  however,  follow 
other  forms  of  curve.  In  the  case  of  unpractised  reaction 
times,  for  example,  the  curve  has  a  highly  marked  skew 
instead  of  a  symmetrical  shape.  For  while  there  is  no 
limit  to  the  possible  length  of  a  reaction  time,  there  is  an 
obvious  limit  to  its  possible  shortness.] 

It  is  particularly  when  we  are  dealing  with  distribu- 
tions in  which  a  few  exceptionally  large  or  small  variates 
are  liable  to  disturb  the  value  of  the  mean,  that  the 
median  presents  a  truer  generalisation  of  the  experi- 
mental results  than  can  be  obtained  from  the  mean.  But 
since  in  very  small  series,  the  median  cannot  be  trusted 
to  give  a  reliable  result,  and  in  sufficiently  large  series, 
such  chance  abnormally  large  or  small  values  do  not  so 
seriously  affect  the  "mean,  the  chief  advantage  of  the 
median  comes  to  lie  in  the  ease  with  which  it  may  be 
determined. 

The  Semi-interquartile  Range. — The  median  represents 
the  value  of  the  central  variate.  It  is  sometimes  useful  to 
know  the  value  of  other  definitely  placed  variates,  e.g.  the 
variates  which  lie  half-way  between  the  median  and  the 
two  extreme  variates.  Two  such  variates,  in  this  particular 
case  termed  quartiles,  are  sometimes  employed  to  indicate 
the  extent  of  variation  in  the  series.  For  this  purpose 
half  the  difference  between  the  quartiles  is  a  useful  ex- 
pression. It  may  be  termed  the  "  semi- interquartile 
range." 

Correlation. — In  psychological  experiment  we  are  often 
concerned  with  determining  the  correlation  between  the 
members  of  two  series  of  measurements.  We  require  to 
know  how  far  the  values  of  variates  in  the  one  series  vary 
concurrently  with  those  in  the  other, — to  what  extent,  for 
9 


130  EXPERIMENTAL  PSYCHOLOGY 

example,  those  individuals  who  give  high  or  low  values  in 
one  particular  mental  trait  also  give  high  or  low  (or  low  or 
high)  values  in  another  trait. 

The  mathematical  symbol,  r,  is  used  to  express  the 
coefficient  of  correlation  between  two  such  traits.  A  very 
widely  useful  formula  for  determining  it  is 


n<r<r 


xy 


where  x  and  y  are  the  deviations  of  the  two  traits  from 
their  mean  in  any  single  individual,  ^(xy)  is  the  sum  of 
these  several  products  of  x  and  y  obtained  for  all  the 
individuals,  crx  and  ay  are  the  standard  deviations  of  the 
two  series  of  measurements,  and  n  is  the  number  of 
individuals  (or  pairs  of  measurements)  examined.  The 
probable  error  of  this  coefficient  of  correlation  is  given 
by  the  expression 

0-6745  (1-r2) 
Vn 

The  possible  values  of  r  range  between  -f  1  and  —  1. 
When  r  is  found  to  be  zero,  the  two  traits  are  absolutely 
devoid  of  correlation;  when  r  =  1,  there  is  perfect  correla- 
tion; when  r=  —  1,  there  is  perfect  inverse  correlation,  — 
that  is  to  say,  every  individual  who  shows  high  values 
in  one  trait  shows  correspondingly  low  values  in  the  other 
trait. 

This  formula  can  be  considerably  simplified  when 
correlation  is  studied,  not,  as  before,  between  two  series 
of  measurements,  but  between  two  series  of  ranks.  The 
simplification  arises  from  the  fact  that  the  variates  are  now 
whole  numbers  from  1  to  n,  and  that  their  standard 
deviation  can  consequently  be  calculated  once  and  for  all. 
The  individuals  are  arranged  in  order  of  rank,  first  as 
regards  one  trait  and  then  as  regards  the  other.  Each 
individual  thus  receives  two  numbers  which  represent  his 


STATISTICAL  METHODS  131 

rank  in  respect  of  each  trait.     Then  it  can  be  shown  that 
the  previous  formula 


where  2(^0  is  the  sum  of  the  differences  between  each  of 
those  pairs  of  numbers.  Either  of  these  formulae  is  a 
much  more  convenient  expression  to  use  (exp.  87). 


BIBLIOGRAPHY. 

K.  Pearson,  The  Grammar  of  Science,  London,  1900,  372-503.  C. 
Davenport,  Statistical  Methods,  New  York,  1904.  E.  L.  Thorndike,  An 
Introduction  to  the  Theory  of  Mental  and  Social  Measurements,  New  York, 
1904.  C.  Spearman,  "  'Footrule'  for  Measuring  Correlation,"  Brit.  Journ. 
of  PsychoL,  1906,  ii.  89  ;  "The  Method  of  'Right  and  Wrong  Cases'  ('Con- 
stant Stimuli'),  without  Gauss's  Formulae,"  ibid.,  227. 


CHAPTEE    XI 
ON    REACTION   TIMES 

Simple  and  Composite  Reactions. — The  reaction  time  of  an 
individual  is  the  interval  that  elapses  between  the  exhibition 
of  a  stimulus  and  his  response  to  it  in  a  prescribed  manner. 
The  reaction  or  response  is  usually  carried  out  by  movement 
of  the  hand  or  lips,  or  by  articulation.  The  quickest  reaction 
is  obtainable  by  an  immediate  response  to  a  previously  known 
and  simple  presentation.  But,  as  we  shall  see,  various  com- 
plexities, as  regards  the  nature  of  the  stimulus  and  the  mode 
of  the  reaction,  may  be  introduced ;  these  materially  modify 
the  length  of  the  reaction  time.  It  is  therefore  convenient 
to  distinguish  "  simple "  from  "  composite  "  (or  complex) 
reactions. 

The  Reduced  Reaction  Time. — The  time  of  the  simple 
reaction  (i.e.  immediate  reaction,  say,  to  a  sound,  a  touch,  or 
a  taste)  is  occupied  partly  in  central  changes  taking  place 
within  the  brain,  and  partly  in  peripheral  changes  which 
occur  outside  it.  Endeavours  have  been  made  to  arrive  at  a 
central  or  "  reduced  "  reaction  time,  by  eliminating  the  time 
that  is  spent  peripherally  in  the  action  of  the  stimulus  on 
the  sensory  organ,  in  the  passage  of  the  afferent  and  efferent 
impulses  along  the  peripheral  nerves  and  within  the  spinal 
cord  to  or  from  the  cerebral  hemispheres,  and  lastly  in 
the  conversion  of  the  efferent  impulse  into  the  muscular 
contraction  which  forms  the  response. 

Unfortunately,   our   knowledge   of   the   speed  of   these 

physiological  processes  is  not  accurate  enough  to  make  such 

132 


REACTION  TIMES 


133 


calculations  reliable.  Yet  they  are  unquestionably  the  cause 
of  certain  differences  in  reaction  time.  Thus  the  slower 
reaction  to  visual  stimuli,  as  compared  with  the  reaction  to 
all  other  (save  painful)  sensory  stimuli,  is  largely  due  to  the 
latent  time  occupied  in  the  development  of  photochemical 
processes  in  the  retina.  And  the  slower  reaction  to  bitter 
than  to  other  tastes  is  doubtless  of  similar  causation.  But, 
in  our  present  ignorance,  we  have  for  the  most  part  to  leave 
these  variable  peripheral  factors  on  one  side,  and  to  confine 
our  studies  of  reaction  times  to  a  comparison  of  the  results 
obtained  under  different  psychical  conditions  from  the 
same  individual  or  from  different  individuals. 


SIMPLE  KEACTIONS. 

Censorial  and  Muscular  Reactions. — Simple  reaction  times 
vary  in  length,  according  to  the  direction  of  the  reagent's 
attention  at  the  moment  of  exhibition  of  the  stimulus.  If 
he  attend  to  the  movement  by  which  he  is  enjoined  to 
respond,  he  generally  reacts  with  considerably  greater  speed 
than  if  his  attention  be  fixed  upon  the  stimulus  which  he  is 
about  to  receive.  The  approximate  differences  between  the 
"  muscular  "  and  the  "  sensorial  "  reactions,  as  these  two 
modes  of  reaction  are  called,  are  here  given : — 


Stimulus. 

Muscular 
Reaction. 

Sensorial 
Reaction. 

Sound 

125-7 

220* 

Light       .         ..     .  .  * 

175°" 

270°- 

Touch      .         .     -  -.' 

lie* 

210a 

Heat 

130°" 

190* 

Cold 

115-7 

150* 

These  figures  express  the  reaction  times  in  one  thousandth 


134  EXPERIMENTAL  PSYCHOLOGY 

parts  of  a  second,  a  unit  for  which  the  symbol  c  is  usually 
employed.1 

There  are  other  particulars  in  which  muscular  reactions 
differ  from  sensorial.  As  a  rule,  they  show  considerably 
less  variability.  Thus,  while  the  mean  variation  (page  124) 
of  the  former  lies  between  6°"  and  9°",  the  mean  variation  of 
the  latter  after  a  corresponding  degree  of  practice  is  from 
24°"  to  28°".  Further,  whereas  "  premature  "  and  "  wrong  "  or 
"  delayed "  reactions  are  liable  to  occur  in  muscular,  they 
are  absent  in  strictly  sensorial  reactions.  A  subject  is  said  to 
react  "  prematurely,"  when  he  inadvertently  reacts  before  the 
stimulus  is  given  or  before  it  can  have  been  received  by  him. 
He  reacts  "  wrongly,"  when  he  responds  to  some  accidentally 
occurring  stimulus  other  than  that  prescribed.  His  reaction 
is  "  delayed,"  when  he  responds  to  the  prescribed  stimulus 
after  having  tended  to  give,  or  after  having  actually  given,  a 
"  wrong  "  reaction  (exp.  89). 

These  several  features  of  the  muscular  and  sensorial 
reaction  are  precisely  those  we  should  expect  to  result  from 
the  differences  in  mental  attitude  of  the  reagent.  In  the 
muscular  reaction,  his  attention  is  confined  to  the  response 
which  is  expected  of  him.  Whether  the  reaction  consists  in 
the  lifting  of  a  finger  or  in  the  speaking  of  a  word,  the 
reagent  has  prepared  the  appropriate  motor  apparatus  for 
action  before  the  arrival  of  the  stimulus.  So  delicately  is 
his  system  balanced,  in  such  readiness  does  he  hold  himself, 
that  he  tends,  as  we  have  seen,  to  "  go  off "  (wrongly  or 
prematurely)  into  movement  on  the  least  provocation ;  the 
vivid  idea  of  movement  uncontrollably  expresses  itself  in 
action.  In  the  sensorial  reaction,  on  the  other  hand,  the 
subject's  attention  is  occupied,  not  with  the  response,  but 
with  an  idea  of  the  impression  which  he  is  about  to  receive. 
Upon  receiving  the  impression,  he  has  first  to  assure  himself 
that  it  is  truly  the  expected  presentation,  whereupon  he 

1  It  is  unfortunate  that  this  time  interval  and  the  standard  deviation  are 
denoted  by  the  same  symbol.     In  practice,  however,  no  confusion  results. 


REACTION  TIMES  135 

reacts  in  the  prescribed  manner.  It  is  not  surprising,  then, 
that  sensorial  are  slower  than  muscular  reactions. 

Psychological  Analysis  of  Reaction  Times. — The  central  or 
reduced  reaction  time  (page  132)  may  be  conceived  as  spent 
in  three  different  mental  processes,  namely,  the  perception 
of  the  impression,  its  apperception,  and  the  volitional 
release  of  the  motor  response.  This  conception,  however, 
is  schematic  instead  of  actual:  philosophical  instead  of 
psychological.  It  is  the  outcome  of  a  priori  notions,  rather 
than  of  an  appeal  to  introspection. 

For  even  in  the  sensorial  reaction  the  distinction  between 
perception  and  apperception  of  the  impression, — that  is, 
'between  vague^  awareness  and  attentive^  notice  of  it, — is 
ill-defined,  while  in  the  practised  muscular  reaction  intro- 
spection fails  to  reveal  any  trace  of  apperception  prior  to 
the  response.  And  as  regards  the  third  of  these  processes, 
the  release  of  the  motor  response,  we  have  to  bear  in  mind 
that  in  either  form  of  reaction  it  is  very  far  from  being 
identical  with  an  ordinary  act  of  volition.  In  the  sensorial 
reaction  the  subject  has  already  decided  how  he  will  react, 
before  he  receives  the  appropriate  impression.  In  the 
muscular  reaction  the  practised  subject  finally  responds 
automatically.  Here  there  is  no  conation ;  nor  is  there  the 
faintest  kinsesthetic  image  of  the  intended  movement.  His 
reaction  becomes,  to  all  intents,  a  reflex, — a  reflex  which  is 
sustained  so  long  as  he  volitionally  preserves  a  psycho- 
logically favourable  disposition  toward  the  experiment. 
Under  such  conditions,  we  are  merely  measuring  the  speed 
with  which  an  already  established  sensori-motor  connection 
finds  expression. 

Effects  of  Practice  and  Fatigue. — The  improvement  in 
the  speed  of  reaction,  effected  by  practice,  is  the  result  of 
a  more  perfect  adaptation  to  the  conditions  of  experiment. 
As  we  might  expect,  it  is  more  noticeable  in  sensorial  than 
in  muscular  reactions ;  indeed,  ultimately,  the  difference  in 
their  times  may  be  considerably  less  than  that  given  in  the 


136  EXPERIMENTAL  PSYCHOLOGY 

previous  table.  At  the  outset  of  any  series  of  experiments, 
the  reagent  is  naturally  far  worse  prepared  in  the  sensorial 
than  in  the  muscular  reaction.  Practice  teaches  him  how 
to  combine  a  certain  readiness  for  movement  with  the  fixation 
of  his  attention  on  the  expected  stimulus,  and  thus  the 
practised  sensorial  approaches  more  and  more  nearly  to 
the  value  of  the  muscular  reaction  time. 

The  effects  of  fatigue  upon  the  reaction  time  are  relatively 
slight.  They  are  chiefly  due  to  the  failure  of  attention. 

Reaction  Movements. — A  careful  study  of  the  variations 
in  finger  pressure,  which  different  reagents  bring  to  bear 
upon  the  reaction  key  during  reaction  experiments,  shows 
individual  differences  of  behaviour,  of  the  significance 
arid  effect  of  which  we  are  at  present  ignorant.  Some 
reagents  habitually  maintain  a  constant  pressure  on  the 
key  while  awaiting  the  expected  signal ;  some  rhythmically 
increase  and  decrease  this  pressure ;  some  gradually  or 
suddenly  decrease  the  pressure  before  the  reaction;  while 
others  gradually  or  suddenly  increase  it  before  the  reaction, 
giving  what  has  been  termed  an  "  antagonistic  "  reaction. 

Natural  Reactions.  —  A  third  form  of  simple  reaction 
has  yet  to  be  mentioned,  in  which  the  subject's  attention 
is  left  altogether  undirected.  He  is  merely  enjoined  to 
react  when  the  stimulus  has  been  exhibited.  This  is  called 
the  "  natural "  reaction ;  its  time  is  commonly  intermediate 
between  the  values  of  the  sensorial  and  muscular  reaction 
times,  the  mental  attitude  now  tending  toward  a  sensorial, 
now  toward  a  muscular  reaction  (exp.  89). 

Individual  Variations. — But  individual  variations  are 
considerable.  Some  reagents  when  left  to  themselves  are 
naturally  prone  to  react  exclusively  in  the  muscular  or 
exclusively  in  the  sensorial  fashion.  Even  after  consider- 
able practice  some  find  it  difficult,  or  indeed  impossible,  to 
change  the  mode  of  their  reaction  from  one  form  to  the 
other.  It  has  been  said  that  some  reagents  give  quicker 
sensorial  than  muscular  reactions. 


REACTION  TIMES  137 

Apparently  it  is  easier  for  the  natural  reaction  of  those 
who  incline  to  the  sensorial  to  be  changed  to  the  muscular 
form  after  continued  practice  at  the  latter,  than  for  the 
natural  reaction  of  those  who  incline  to  the  muscular  to  be 
changed  to  the  sensorial  form  after  practice  at  sensorial 
reactions. 

Attempts  have  been  made  to  correlate  these  individual 
differences  in  reaction  with  individual  differences  in  imagery. 
A  person  in  whom  motor  imagery  predominates  would 
incline,  so  it  has  been  said,  to  the  muscular  reaction,  while 
a  difficulty  in  reacting  sensorially  would  result  from  his 
inability  to  preserve  that  clear  image  of  the  expected 
stimulus,  which  persons  endowed  with  vivid  auditory,  visual 
or  other  imagery,  could  attain.  Although  the  evidence  in 
favour  of  such  a  connection  between  type  of  reaction  and 
of  imagery  is  somewhat  conflicting,  it  may  reasonably  be 
supposed  that  in  a  slight  degree  they  are  correlated. 

Influence  of  Age  and  Race. — In  childhood  and  in  old 
age,  natural  reaction  times  are  longer  and  their  mean 
variations  are  larger  than  in  intervening  periods  of  life,  a 
result  which  is  doubtless  to  be  attributed  to  a  more  difficult 
maintenance  of  the  favourable  attitude  of  readiness,  owing 
to  wandering  attention,  lack  of  agility,  and  the  like.  The 
racial  differences  that  exist  in  reaction  times  are  largely 
the  outcome  of  similar  psychological  factors,  determined  by 
habits  of  life  and  possibly  by  some  obscure  racial  tendency 
to  react  rather  in  the  sensorial  than  in  the  muscular  fashion, 
or  vice  versd. 

Tke  Personal  Equation.  —  Observations  on  individual 
differences  of  reaction  time  throw  light  on  the  nature  of 
the  so-called  "  personal  equation,"  which  astronomers  have 
long  recognised  and  taken  into  account,  in  the  comparison 
of  transit  observations  made  by  different  observers.  If  to 
the  individual  differences  in  reaction  time  we  add  the 
alterations  of  apparent  time  order,  produced  by  change  of 
the  field  of  attention  (page  317),  we  can  fairly  picture  the 


138  EXPERIMENTAL  PSYCHOLOGY 

psychological  causes  of  those  errors  to  which  the  astronomer 
is  liable,  when  he  registers  the  moments  at  which  a  moving 
star  crosses  the  parallel  lines  fixed  within  his  telescope, 
either  by  registering  those  moments  or  by  counting  the 
beats  of  a  clock. 

Other  Determinants  of  Reaction  Times. — We  have  already 
pointed  out  (page  132)  that  the  reaction  time  must  partly 
depend  upon  the  time  occupied  at  the  peripheral  sense 
organ  in  developing  the  afferent  nervous  impulse.  More- 
over, it  is  conceivable  that  all  sensations  do  not  travel  by 
equally  direct  or  easy  paths  from  the  periphery  to  the 
centre.  This  may  perhaps  help  to  explain  the  longer  latent 
period  in  the  development  of  sensations  of  pain,  compared 
with  those  of  touch. 

The  reaction  time  varies  in  the  case  of  different  sensa- 
tions of  the  same  sense  organs.  It  is  generally  shorter  for 
high  than  for  low  tones,  and  shorter  for  noises  than  for 
tones.  Such  variations  are  probably  due  less  to  peripheral 
physiological  factors  than  to  factors  of  a  more  complex 
"  psychological "  order. 

The  strain  on  the  attention,  produced  by  the  use  of 
extremely  feeble  stimuli,  leads  to  prolongation  of  the  re- 
action time.  Very  faint  presentations,  whatever  be  their 
nature,  have  been  said  to  yield  a  uniform  reaction  time  of 
about  33(K  As  might  be  expected,  reaction  times  are 
lengthened  and  their  mean  variation  is  increased,  when  the 
stimuli  vary  irregularly  from  reaction  to  reaction  in  in- 
tensity or  quality,  or  when  the  reagent  never  knows  when 
he  may  expect  the  stimulus.  The  quickest  reactions  are 
obtained,  when  a  warning  signal  precedes  the  exhibition  of 
the  stimulus  by  an  interval  of  between  one  and  two  seconds. 

COMPOSITE  EEACTIONS. 

Recognitive  and  Discriminative  Reactions. — There  are 
various  means  by  which  the  apperceptive  and  volitional 


REACTION  TIMES  139 

factors,  vaguely  shadowed  in  the  simple  sensorial  reaction, 
may  be  brought  still  more  to  the  fore.  The  reagent  may  be 
instructed  not  to  react  until  he  has  "  recognised  "  in  full 
detail  the  presentation.  Or  a  variety  of  stimuli,  with  which 
the  reagent  is  already  more  or  less  familiar,  may  be  used, 
and  he  may  be  enjoined  to  "  discriminate "  between  the 
particular  stimulus  presented  and  the  other  possible  stimuli 
before  he  reacts.  By  these  means,  the  reaction  time  is 
prolonged  by  a  period  varying  from  30°"  to  upwards  of  100°", 
according  to  the  difficulty  of  the  mental  process  involved 
(exp.  90). 

Choice  Reactions. — The  complexity  of  the  experiment 
may  be  further  increased  by  employing  two  stimuli  and  by 
enjoining  the  subject  (i.)  to  react  if  the  one  stimulus 
appears  and  not  to  react  for  the  other,  or  (ii.)  to  react 
in  one  way  (e.g.  lifting  one  finger  or  one  hand)  for  the  one 
stimulus  and  to  react  in  another  way  (e.g.  lifting  another 
finger  or  the  other  hand)  for  the  other  stimulus.  The 
subject  has  here  not  only  to  recognise  and  to  discriminate 
between  the  individual  presentations,  but  to  behave  differ- 
ently towards  each  of  them. 

Such  "  choice  "  reactions  are  longer  by  about  70°"  than 
the  corresponding  "  discriminative  "  reactions.  The  reaction 
becomes  still  further  delayed,  the  greater  the  number  of 
possible  stimuli  and  of  modes  of  reaction  introduced. 
When  they  are  ten  in  number,  when,  for  example,  a 
different  finger  of  one  or  other  hand  must  react  for 
each  of  ten  different  possible  presentations,  the  average 
choice  reaction  time  has  been  found  to  exceed  the  discrimi- 
native reaction  time  by  about  400°"  (exp.  90). 

Naming  and  Reading  Reactions. — Instead  of  reacting  by 
finger  movement,  the  reagent  may  be  required  to  name  or 
to  read  the  presented  stimulus ;  in  such  experiments  the 
act  of  articulation  becomes  the  reaction  movement.  It 
appears  that  while  the  time,  occupied  in  the  reading  of 
short  words  (about  390°"),  is  not  sensibly  different  from  the 


140  EXPERIMENTAL  PSYCHOLOGY 

choice  reaction  time  for  two  words,  the  time  occupied  in 
naming  colours  (about  550°")  is  distinctly  longer  than 
the  choice  reaction  time  between  two  colours  (about 
350»). 

All  these  forms  of  composite  reactions  are  far  more 
readily  executed  by  the  subject  who  naturally  tends  to  the 
sensorial  variety  of  simple  reaction.  The  individual,  who 
would  react  muscularly,  must  needs  learn  to  control  his 
reaction  movement  until  the  presentation  has  been  recog- 
nised or  discriminated,  or  until  the  appropriate  movement 
has  been  decided  upon. 

Psychological  Analysis  of  Composite  Reactions. — We  have 
already  pointed  out  (page  135)  how  the  volitional  element 
in  the  simple  reaction  differs  from  a  volitional  act  in  the 
ordinary  sense  of  the  term.  So,  too,  the  processes  of  recog- 
nition, discrimination,  and  choice  in  composite  reactions  are 
very  far  from  being  comparable  with  those  processes  in 
ordinary  life.  To  some  extent  the  reagent  is  already 
familiar  with  the  demands  about  to  be  made  on  him,  before 
any  individual  reaction  is  carried  out.  If,  for  example,  he 
be  required  to  discriminate  between  two  colours  or  between 
two  sounds,  he  will  have  already  seen  or  heard  these  stimuli 
in  previous  reactions.  Or  again,  if  he  have  to  choose 
between  various  modes  of  reaction,  there  very  soon  ceases 
to  be  a  genuine  deliberate  choice  between  the  ideas  of 
possible  movements  which  occur  to  him  when  the  stimulus 
is  presented.  After  a  little  experience  he  immediately 
recognises  the  idea  corresponding  to  the  correct  movement, 
just  as  he  would  directly  and  without  the  intervention  of 
choice  identify  the  correctness  of  the  revived  image  of  a 
word  which  he  had  previously  learned  in  association  with 
another  word.  Thus  choice  reaction  times  give  place  to 
what  in  memory  we  call  reproduction  times  (page  154). 
Finally,  with  sufficient  practice  the  reagent  plays  upon  the 
reaction  keys  as  automatically  as  an  accomplished  pianist 
comes  to  read  a  piece  of  music  set  before  him  at  the  piano- 


REACTION  TIMES  141 

forte,  each  impression  unconsciously  releasing   the  appro- 
priate movement. 

ASSOCIATIVE  REACTIONS. 

In  point  of  fact,  all  reactions  involve  various  processes 
of  association,  but  the  <l  associative  reaction "  is  a  term 
given  to  the  experiment  in  which  the  reagent  has  to  respond 
to  the  word  stimulus  by  another  word  (or  idea)  which  is 
somehow  associated  with  it  (exp.  91). 

Forms  of  Associative  Reactions. — The  association  may  be 
"  free  "  or  "  wholly  "  or  "  partly  constrained."  It  is  free, 
when  the  subject  replies  to  the  stimulus  (a  word  heard  or 
read)  by  giving  the  first  idea  that  occurs  to  him.  It  is 
wholly  constrained,  when  practically  only  one  correct 
response  can  be  returned  by  him  ;  thus  he  may  be  asked  to 
translate  a  foreign  word  presented  to  him,  to  perform  a 
mathematical  calculation,  or  to  name  the  capital  of  a  given 
country.  The  association  is  partly  constrained,  when,  for 
instance,  a  generic  word  is  exhibited  and  the  subject  is 
required  to  reply  with  a  specific  example,  or  when  an 
adjective  is  exhibited  and  the  reagent  is  required  to  name  a 
noun  to  which  the  epithet  is  appropriate. 

Association  Times. — Associative  reactions  are  on  the 
average  about  'ZOO  longer  than  reactions  in  which  the 
presented  word  has  merely  to  be  articulated.  But  there 
are  wide  variations  in  the  times  of  individual  associative 
reactions.  They  may  range  from  700°"  to  over  1400°", 
according  to  the  nature  of  the "  association  and  the  mental 
condition  of  the  reagent ;  they  are  generally  longer  in  free 
than  in  constrained  associations.  In  a  later  chapter  we 
shall  deal  with  the  classification  of  associations,  and  we 
shall  show  how  the  times  of  wholly  constrained  associations 
(in  other  words,  how  reproduction  times)  may  be  used  as  a 
test  of  the  strength  of  associations. 

Mathematical  Analysis  of  Reaction  Times. — Certain  psy- 


142  EXPERIMENTAL  PSYCHOLOGY 

chologists  have  not  hesitated  to  subtract  the  times  of  these 
various  forms  of  reaction  from  one  another,  thinking  thereby 
to  arrive  at  the  speed  of  the  special  mental  processes  in- 
volved in  the  different  reactions.  Thus,  they  have  believed 
it  possible  to  determine  the  time  taken  up  in  recognition  by 
subtracting  the  mean  simple  reaction  time  from  the  mean 
recognitive  reaction  time,  and  to  determine  the  time  taken 
up  in  choice  by  subtracting  the  mean  discriminative  re- 
action time  from  the  mean  choice  reaction  time,  and  so  on. 
But  such  mathematical  treatment  of  psychological  data  is 
utterly  indefensible,  unless  it  be  grounded  on  most  careful 
introspection.  We  must  recognise  that  the  formal  schemes 
of  abstract  thought,  dictated  by  logic  or  by  mathematics, 
are  not  necessarily  followed  in  the  concrete.  And  even 
when  separate  procedures  of  the  mind  may  be  introspect- 
ively  separated  from  a  complex  piece  of  mental  behaviour, 
it  by  no  means  follows  that,  when  a  third  psychical  process 
is  experimentally  added  to  two  others,  these  latter  preserve 
their  nature  and  duration,  undisturbed  by  the  entry  of 
the  third.  On  the  contrary,  there  is  a  priori  every  reason 
to  suppose  that  they  will  be  modified. 

The  Physiological  Aspect  of  Reaction  Times. — We  have 
to  guard  against  the  common  inference  that  as  any  piece  of 
behaviour  becomes  more  purely  reflex,  the  nervous  paths 
traversed  must  necessarily  become  more  and  more  confined 
to  the  lower  parts  of  the  brain  and  to  the  spinal  cord. 
There  is  no  evidence  to  show  that  an  acquired  habit  ever 
quits  the  motor  cortical  paths  which  from  the  outset  it  had 
taken.  Were  it  the  case,  were  the  acts  handed  over  from 
cortical  to  subcortical  and  finally  to  spinal  areas  as  they 
become  more  habitual,  we  might  expect  the  effects  of 
continued  practice  with  which  we  meet  to  be  interrupted 
instead  of  gradual.  And  we  should  expect  that  where, 
through  injury  or  disease,  the  cortical  motor  centres  are  put 
out  of  action  and  the  lower  nuclear  centres  remain  in  func- 
tion, habitual  acts  would  still  be  capable  of  performance ; 


REACTION  TIMES  143 

but  this  is  not  found  to  be  the  case.  On  the  contrary,  the 
evidence  is  strongly  in  favour  of  the  view  that  the  conscious 
or  unconscious  performance  of  an  action  is,  on  the  whole, 
correlated  with  the  degree  of  difficulty  of  its  performance, 
and  that  both  the  degree  of  consciousness  and  the  difficulty 
of  performance  are,  in  part  at  least,  the  expression  of  the 
resistance  which  is  offered  at  the  centres  to  the  passage 
of  the  nervous  impulses,  and  which  becomes  reduced  by 
adequate  practice. 

BIBLIOGRAPHY. 

G.  Buccola,  La  Legge  del  Tempo  nei  Fenomeni  del  Pensiero,  Milano,  1883. 
L.  Lange,  "  Neue  Experimente  iiber  d.  Vorgang  d.  einfachen  Reaction  auf 
Sinneseindriicke,"  Philosoph.  Stud.,  1888,  iv.  479.  L.  W.  Stern,  Ueber  d. 
Psychol.  d.  individuellen  Di/erenzen,  Leipzig,  1900,  103.  N.  Aleehsieff, 
"Ueber Reaktionsversuche bei Durchgangsbeobachtungen," Philosoph.  Stud., 
xvi.  1900,  15.  C.  S.  Myers,  Reports  of  the  Cambridge  Anthropol.  Expedition 
to  Torres  Straits,  Cambridge,  1903,  ii.  205.  W.  G.  Smith,  ''Antagonistic 
Reactions,"  Mind,  1903  (N.S.),  xii.  47.  W.  Wundt,  Grundziige  d.  physiol. 
Psychol.,  Leipzig,  5te  Aufl.,  1903,  iii.  315.  C.  H.  Judd,  C.  N.  M'Allister, 
and  W.  M.  Steele,  "Analysis  of  Reaction  Movements,"  Psychol.  Rev., 
Monograph  Suppl.,  1905,  vii.  No.  1,  141.  E.  B.  Titchener,  Experimental 
Psychol.,  New  York,  1905,  i.  exp.  xxvi. ;  ii.  chap.  3.  A.  Aliotta,  LaMisura 
in  Psicologia  sperimentale,  Firenze,  1905.  R.  Bergmann,  "  Reaktionen  auf 
Schalleindriicke,  nach  d.  Methoded.  Haufigkeitskurven  bearbeitet, "  Psychol. 
Stud.,  1905,  i.  179.  H.  J.  Watt,  " Experimented  Beitr.  zu  einer  Theorie 
d.  Denkens,"  Arch.f.  d.  ges.  Psychol.,  1905,  iv.  289.  B.  Edgell  and  W.  L. 
Symes,  "The  Wheatstone  -  Hipp  Chronoscope, "  etc.,  Brit.  Journ.  of 
Psychol.,  1906-8,  ii.  58,  281. 


CHAPTEE    XII 

ft 
ON   MEMORY1 

The  Perseverance  of  Experiences. — The  tendency  of  past 
experiences  to  reproduce  themselves  spontaneously  con- 
stitutes their  tendency  to  "  perseverance."  The  "  running  " 
of  a  tune  in  the  head,  the  persistent  revival  of  a  painful  scene, 
the  reappearance  of  striking  events  of  the  day  just  before 
the  onset  of  sleep, — each  is  an  instance  of  perseverance. 
The  perseverance  tendency  shows  considerable  variation  in 
different  individuals.  It  is  generally  strongest  immediately 
after  the  original  presentation,  and  with  the  onset  of  fatigue. 
Its  strength  increases  with  the  attention  given  to  the 
presentation  and  with  the  number  of  repetitions  of  the 
latter  (exps.  92,  93). 

Association. — Apart  from  perseverance,  experiences  tend 
to  reproduce  themselves  by  virtue  of  their  "association." 
If  an  experience  a  be  associated  with  an  experience  Z>,  the 
recurrence  of  a  tends  to  reproduce  the  experience  I  in  con- 
sequence of  the  association  between  them.  Ceteris  paribus, 
when  the  association  is  weak,  the  tendency  to  reproduction 
is  slight ;  when  it  is  strong,  the  reproduction  tendency  is 
great.  (The  revival  of  presentations  is  dependent,  then,  both 
on  perseverance  and  on  association  strength.) 

Memory  Images. — Such  reproduced  experiences,  when 
sufficiently  definite,  involve  "memory^  images."  They 
contain  most  of  the  characters  of  the  original  experiences ; 
an  auditory  memory  image,  for  example,  being  of  high  or 

1  See  footnote  to  Chapter  III. 
144 


MEMORY  145 

of  low  pitch,  a  visual  memory  image  being  feeble  as  in 
the  image  of  a  candle,  or  intense  as  in  the  image  of  an 
incandescent  light.  But  apart  from  such  differences, — apart, 
for  instance,  from  variations  in  intensity, — memory  images 
also  vary  in  vividness.  The  more  vivid  a  memory  image  is, 
the  more  will  it  attract  and  strike  the  attention,  and  the 
nearer  will  it  resemble  the  reality  of  an  original  experience. 
Under  ordinary  conditions,  however,  there  is  always  some- 
thing wanting  in  the  re-presentation  of  memory  images, 
which  prevents  them  from  being  confused  with  presentations 
(exps.  92,  93). 

The  Kinds  of  Imagery. — There  are  wide  individual  differ- 
ences in  the  vividness  of  memory  images.  Some  persons 
declare  that  the  memory  images  which  they  can  produce  at 
will  are  as  vivid  as  those  which  occur  immediately  after  the 
original  presentation ;  but  such  experiences  are  uncommon. 

It  is  well  known  that  various  kinds  of  imagery  are 
developed  to  very  different  extents  in  different  people. 
Whereas  some  possess  especially  vivid  visual  imagery, 
others  excel  in  auditory  or  in  kin£esthetic  imagery.  Some 
can  imagine  a  scene  in  all  its  original  colouring  ;  the  visual 
memory  images  of  others  are  entirely  colourless  or  almost 
wanting.  Some  dream  vividly  of  tastes  and  odours,  some 
can  call  to  mind  tactual  experiences ;  whereas  in  others  gusta- 
tory, olfactory,  and  tactual  images  are  very  rare.  Probably 
no  one  is  absolutely  devoid  of  any  of  these  kinds  of  imagery, 
unless  he  have  been  deprived  from  birth  (or  within  his 
first  few  years)  of  the  sensory  experiences  on  which  they 
respectively  depend.  All  our  evidence  goes  to  show  that 
exercise,  dependent  on  interest  and  education  and  heredity, 
plays  the  most  important  part  in  selecting  the  predominant 
type  of  imagery. 

In  the  case  of  most  of  us,  our  everyday  images  are  a 
blend  of  several  kinds  of  imagery.  Several  methods,  how- 
ever, have  been  employed  in  the  laboratory  to  determine 
the  predominant  kind  in  a  given  individual.  According  to 

10 


146  EXPERIMENTAL  PSYCHOLOGY 

one  method,  he  writes  down  a  list,  first  of  objects  which 
possess  well-marked  colours,  next  of  objects  which  are 
characterised  by  sounds,  and  so  on  ;  devoting  the  same  time, 
e.g.  five  minutes,  to  each  of  these  groups.  Those  individuals 
in  whom,  for  example,  auditory  imagery  is  most  strongly 
developed  will  be  found  to  write  down  a  far  longer  list  of 
words  which  have  sounds  connected  with  them  than  will  be 
done  in  the  same  time  by  folk  in  whom  this  imagery  is  less 
developed. 

Another  method  of  testing  the  kind  of  imagery  is  by 
learning.  Several  groups  of  twelve  letters  are  prepared, 
each  twelve  being  printed  on  a  separate  card  in  three 
vertical  rows.  In  a  certain  number  of  experiments,  the 
twelve  letters  are  twice  read  through  by  the  subject,  in 
some  experiments  with  or  without  movements  of  his  lips,  in 
others  silently  or  aloud.  In  other  experiments,  the  letters 
are  not  exposed,  but  are  only  heard  as  they  are  read  by  the 
experimenter.  The  reading  of  the  letters  is  regulated  so  as 
to  occupy  a  constant  time,  say  ten  seconds.  At  a  given 
time,  say  twenty  seconds,  after  each  exhibition  of  a  card  of 
twelve  letters,  the  latter  are  reproduced  by  the  subject,  and 
a  careful  record  is  taken  of  the  percentage  of  his  correct 
replies  and  of  the  nature  of  his  mistakes,  according  to  the 
conditions  of  learning. 

The  more  pronounced  be  the  visual  imagery  of  the 
individual  and  the  less  pronounced  his  auditory  imagery,  the 
more  successfully  will  he  reproduce  what  he  has  silently 
seen,  rather  than  what  he  has  heard.  On  the  other  hand, 
auditory  imagery  will  be  most  assisted  by  hearing  the  letters 
or  by  declaiming  them  while  reading;  and  kinsesthetic 
imagery  will  be  most  assisted  by  movements  of  the  lips  and 
glottis.  The  imagery  of  the  predominantly  visual  type  will 
be  little  affected,  if,  while  reading  the  letters,  he  utter  a 
prolonged  vowel,  say,  ah.  The  imagery  of  the  auditory 
type,  on  the  contrary,  will  be  thereby  placed  at  a  distinct 
disadvantage. 


MEMORY  147 

Some  instructive  results  are  also  afforded  by  an  exa- 
mination of  the  mistakes  in  reproduction  made  by  different 
individuals.  In  the  predominantly  visual  type-letters  of 
like  appearance,  in  the  predominantly  auditory  type  letters 
of  like  sound  are  liable  to  be  reproduced  in  the  place  of 
those  actually  presented.  Individuals,  in  whom  kinsesthetic 
imagery  is  especially  developed,  feel  a  tendency  to  speak 
or  to  sing  during  reproduction ;  they  confuse  letters  with 
others  requiring  similar  articulation.  Those  in  whom 
auditory  imagery  is  developed  hear  the  letters  sounding 
during  reproduction,  and  for  this  reason  often  tend  to  make 
fewer  mistakes  with  vowels  than  with  consonants.  In 
those  whose  imagery  is  of  the  visual  kind,  on  the  other 
hand,  consonants  are  revived  just  as  easily  as  vowels ;  an 
image  of  the  entire  card  of  letters  stands  vividly  before 
them,  from  which  they  can  reproduce  the  letters  diagonally 
or  in  any  other  order  as  requested.  This  can  only  be 
achieved  imperfectly  and  with  the  greatest  difficulty  in  the 
case  of  the  auditory  type  of  imagery,  where,  of  course,  the 
letters  are  revived  successively. 

Those  in  whom  one  or  other  of  these  three  kinds  of 
imagery  is  especially  developed,  are  sometimes  called 
"visiles,"  "audiles,"  or  "motiles."  Cases  are  on  record  in 
which,  owing  to  organic  or  to  merely  functional  disorder,  one 
kind  of  imagery  completely  vanishes,  and  another  is  success- 
fully cultivated  to  take  its  place.  So,  too,  in  the  course  of 
prolonged  experimental  work  on  memory,  subjects  have  noted 
that  the  kind  of  imagery  which  they  employ  is  liable  to 
change.  From  general  inquiries  it  appears  that  painters  and 
women  have  especially  vivid  visual  imagery,  that  imagery  is 
particularly  vivid  in  childhood,  and  that  the  visual  type  is  rare 
among  men  who  are  engaged  in  scientific  work.  But  the  data 
at  our  command  are  not  precise  enough  for  further  generalisa- 
tions. We  know  little  or  nothing  of  the  minor  individual 
variations  in  imagery  which  must  in  part  be  responsible  for 
proficiency  in  special  memories,  e.g.  for  names,  faces,  or 


148  EXPERIMENTAL  PSYCHOLOGY 

numbers.  Nor  have  we  any  introspective  data  of  value 
from  those  rare  individuals  who  declare  that  they  are 
totally  devoid  of  any  form  of  imagery. 

Memory  After-images. — A  distinction  has  been  sometimes 
drawn  between  memory  images  and  "  memory  after-images." 
This  is  based  principally  on  the  time,  the  course,  and  the 
vividness  of  their  respective  appearance.  Memory  after- 
images, sometimes  called  "  primary  "  memory  images,  occur 
almost  immediately  after  the  attentive  perception  of  an 
object,  and  are  usually  far  more  vivid  than  the  subsequent 
memory  images. 

Care  must  be  taken  not  to  confuse  memory  after-images 
with  the  positive  after-images,  which  are  due  to  persistent 
sensory  processes.  When  we  contrast  these  images  in 
the  case  of  vision,  we  note  that  the  sensory  after-image 
follows,  while  the  memory  after-image  is  independent  of 
movements  of  the  eyes,  and  that  the  sensory  after-image 
is  projected  externally  while  the  memory  after-image 
is  located  internally;  and  there  are  other  less  striking 
differences  (exps.  92,  93). 

Like  many  sensory  after-images,  however,  the  memory 
after-image  undergoes  fluctuations.  An  appreciable  interval 
elapses  before  it  first  appears,  it  rapidly  gains  its  maximal 
intensity  and  vividness,  after  which  it  gradually  fades.  In 
the  case  of  the  visual  image  it  waxes  and  wanes  perhaps 
several  times  per  second ;  while  from  time  to  time  it 
vanishes  altogether  for  a  few  seconds.  Finally,  it  disappears, 
after  a  period  which  varies  usually  from  thirty  seconds  to 
several  minutes,  according  to  the  individual  and  the  nature 
and  duration  of  the  presentation.  These  fluctuations  are 
readily  studied  by  careful  introspective  analysis,  and  by  the 
use  of  suitable  apparatus  whereby  the  observer  can  by  pre- 
arranged signals  record  the  changes  in  periodicity  or  vivid- 
ness of  the  memory  image  (cf.  exp.  148). 

[Experiments  on  the  Fading  of  Images. — It  is  a  familiar 
fact  that  as  time  goes  on,  the  memory  image  of  a  past 


MEMORY  149 

experience  which  we  are  able  to  revive  becomes  increasingly 
fainter.  Attempts  have  been  made  to  study  this  process 
of  fading  by  various  experimental  methods.  In  some  in- 
vestigations the  original  presentation  has  been  a  given 
colour  or  a  given  brightness,  in  others  a  line  of  definite 
length,  or  a  geometrical  figure,  or  a  tone  of  known  pitch, 
or  a  given  time  interval,  or  a  touch  upon  the  skin  in  a 
definite  spot.  After  varying  intervals  of  time,  the  observer 
is  asked  to  reproduce  this  presentation;  or  a  series  of 
presentations  is  successively  or  simultaneously  given,  some 
of  which  are  nearly,  and  others  are  exactly,  like  the  original. 
The  accuracy  with  which  the  observer  is  able  to  reproduce 
or  to  identify  the  original  presentation,  is  taken  as  a  measure 
of  the  vividness  of  his  memory  image  after  the  lapse  of  a 
given  interval  of  time. 

These  investigations  have  led  to  the  most  discordant 
results.  Some  workers  have  found  a  fairly  simple  log- 
arithmic ratio  between  the  accuracy  of  memory  and  the 
length  of  time  elapsing  since  the  original  presentation, 
while  others  have  failed  to  find  evidence  for  any  such 
relation.  Some  have  observed  a  tendency  to  error  of 
judgment  in  one  direction,  while  the  precisely  opposite 
tendency  has  been  maintained  by  others. 

These  and  other  discrepancies  are  but  the  natural 
result  of  the  use  of  non-comparable  methods,  all  directed  to 
the  investigation  of  the  same  problem,  despite  the  fact  that 
they  involve  the  action  of  different  psychical  processes,  or 
of  the  same  processes  in  different  degrees  of  activity.  In 
some  of  the  experiments,  for  example,  to  which  we  have 
just  referred,  the  observer  is  required  to  produce  his  original 
experience,  while  in  others  he  has  merely  to  choose  and  to 
recognise  the  original  among  a  number  of  later  presentations 
which  are  given  to  him  by  the  experimenter. 

But  even  within  the  limits  of  any  one  of  the  above- 
mentioned  experiments,  there  are  important  differences 
in  the  behaviour  of  different  observers,  or  of  the  same 


150  EXPERIMENTAL  PSYCHOLOGY 

observer  at  different  times ;  all  of  which  require  the  most 
careful  investigation  and  differentiation,  before  the  data 
obtained  can  be  employed  to  throw  light  on  the  course  and 
nature  of  the  memory  image.  One  observer,  for  instance, 
will  preserve  a  relatively  passive  attitude  in  the  interval 
between  the  original  presentation  and  its  reproduction ; 
another  will  use  every  conceivable  device  to  keep  the 
memory  image  (or  some  semblance  of  it)  before  him  during 
that  interval.  There  will  be  chance  and  individual  varia- 
tions of  attention.  There  will  be  apparent  differences, 
which  are  only  accidental  owing  to  insufficient  data  and 
excessive  variability.] 

[Comparison  without  Imagery. — A  yet  more  important 
objection  may  be  urged  against  these  experiments,  namely, 
the  neglect  of  the  fact  that  identification  is  possible  in  the 
absence  of  any  recognisable  memory  image.  An  experience 
may  be  judged  as  identical  with  or  different  from  a  previous 
experience,  not  on  the  ground  of  a  careful  comparison  of 
the  memory  image  of  the  former  with  the  latter  experience 
(or  its  memory  image),  but  because  the  general  "  situa- 
tion "  is  felt  to  be  the  same  or  different  in  the  two  cases. 
The  transition  from  identification  by  the  aid  of  definite 
imagery  to  identification  owing  to  a  vague  resembling 
situation  occurs  to  nearly  every  one  who  has  had  practice  in 
such  experiments.  From  the  former  method,  involving  a 
laborious  and  often  uncertain  judgment,  he  passes  insensibly 
to  the  latter,  whereupon  his  verdict  flashes  forth  spon- 
taneously and  unhesitatingly.] 

[Influence  of  Time  and  Speech  on  Imagery. — There  can, 
however,  be  no  doubt  that  in  course  of  time  the  influence 
both  of  the  situation,  as  we  have  called  it,  and  of  the  true 
memory  image  becomes  impaired,  at  first  rapidly,  later  more 
slowly,  and  that  they  suffer  in  loss  of  accuracy  and  vivid- 
ness. There  are  certain  effects  of  time  upon  the  memory 
image  which  are  specially  noteworthy.  It  seems,  for 
instance,  that  when  we  retain  the  memory  image  of  a  tone, 


MEMORY  151 

we  are  apt  voluntarily  to  sharpen  it,  owing  to  a  belief  that 
as  it  grows  dimmer  it  is  also  growing  flatter  (page  32). 
We  are  also  liable  to  vary  the  shade  of  a  given  grey  in  the 
effort  of  preserving  a  memory  image  of  that  presentation, 
for  we  may  have  judged  the  original  grey  by  a  vague  verbal 
standard,  and  we  may  attempt  to  revive  it  by  recollecting 
that  the  original  presentation  was  considered  "  bright lf  or 
"  rather  dull "  or  "  very  dark."  Such  verbal  standards  or 
symbols  clearly  play  a  most  important  part  in  our  daily 
life.  Indeed,  they  may  prevent  a  given  impression  from 
ever  passing  into  complete  oblivion.  Thus  at  the  time  of 
presentation  we  are  wont  to  note  that  a  friend's  hair  is 
silvery  white,  or  that  an  oblong  is  the  size  of  a  domino. 
Ultimately  we  come  to  fix  very  many  of  our  experiences  by 
means  of  such  purely  verbal  associations. 

Indeed,  the  more  cultureS  the  individual,  the  more  he 
comes  to  rely  on  words,  especially  on  abstract  words, 
rather  than  on  the  imagery  of  concrete  objects.  To  such 
an  extent  may  this  use  of  internal  language  predominate, 
that  the  individual  ultimately  almost  completely  loses 
the  more  elementary  forms  of  visual,  auditory,  or  other 
imagery.] 

The  Classification  of  Associations.-(-ThQ  revival  of  memory 
images,  save  in  so  far  as  it  is  the  result  of  perseverance,  is 
dependent  on  association,  j  The  nature  of  the  associations 
between  our  experiences  has  been  studied  by  various  ex- 
perimental methods.  According  to  one  method,  which  we 
may  term  the  "  serial  method,"  the  subject  begins  with  a  given 
word,  writes  clown  the  word  which  it  immediately  suggests, 
then  writes  down  the  word  immediately  suggested  by  the 
second  word,  and  so  on  for  a  given  length  of  time. 
According  to  another,  the  "  reaction  method,"  a  printed  word 
is  exposed,  whereupon  the  subject  instantly  declares  the 
first  word  or  idea  which  occurs  to  him ;  and  a  large  number 
of  replies  are  collected  by  successively  exposing  a  long 
series  of  such  words.  A  study  of  the  pairs  of  associated 


152 


KXl'KKIMI'NTAL  PSYCHOLOGY 


words,  llius  obUim'il,  has  Unl   to   various  classifications   of 
associations,  of  which  the  following  is  an  example: — 

f'oo-ordination  e.g.  baly — infant. 
suporord ination  e.g.  soldier — man. 
siil.,.nlination  e.g.  man-soldier. 


Similarity 


I  Contrast 


rin  letters  or 
an  sound      '    syllables 


e.g.  peace— war. 


Contiguity 


in  time 


SjtJU'l' 


I    M  i i.i i.  .•  e.g.  port — porter, 

I  in  rhyme  e.g.  fight— kite. 

f  causal  e.g.  lightning — thunder. 

t  verbal  e.g.  one— two, 

snow— snowball . 
e.g.  handle — lock. 


In  these  reaction  experiments,  "  false "  associations 
occasionally  occur,  a  word  being  returned  in  which  no 
association  whatever  can  be  traced  with  the  presented  word. 
By  experimentally  varying  the  interest  of  the  subject  at  the 
moment  the  word  is  exhibited,  very  different  answers  can  be 
obtained  from  him  at  different  times  to  the  same  word. 
Sometimes  the  subject  merely  repeats  the  presented  word, 
or  he  reacts  with  a  word  which  has  been  already  presented 
to  or  returned  by  him  in  a  previous  reaction.  Sometimes, 
owing  to  perseverance,  a  word  is  returned  or  tends  to  return 
again  and  again  (exp.  94). 

The  classification  of  answers  is  possible  only  after  an 
appeal  to  the  subject's  introspection.  For  example,  the 
associated  pair  might — right  may  be  the  result  of  associa- 
tion by  contrast  in  meaning,  by  similarity  in  sound,  or  by 
contiguity  in  time,  according  as  he  was  influenced  by 
the  meaning,  the  rhyme,  or  the  proverb.  So,  too,  the 
association  snow — snowball  may  depend  on  contiguity  in 
time  (verbal  or  causal)  or  space,  or  on  similarity  in  sound 
or  moaning.  In  spite  of  these  difficulties,  however,  such 
systems  of  classification  have  been  proved  to  have  practical 
value.  The  percentages  of  answers  returned  for  the  various 


MKiMOKY  153 

classes  ;irc   found  to  be   influenced    by  fatigue,  l»y  drugs,  and 

by  pathological  disorders  of  the  nervous  system.    Broadly 

speaking,  the  effect  of  these  conditions  is  to  produce  an 
increase  in  the  proportion  of  associations  by  similarity  in 
sound  and  a  decrc.-ise  in  the  proportion  of  those  by  similarity 
in  meaning. 

h'.r.fwrininit  in  /snrnhii/.—  \[,  is  essential  that  the 
experimental  conditions  be  as  simple  as  possible  at  the. 
outset  of  any  psychological  investigation.  And,  in  the  case 
of  the  preliminary  experiments  in  learning,  it  is  particularly 
desirable  that  the  words  or  objects  which  are,  to  be  impressed 
be  of  the  simplest  nature,  so  that  the  conditions  remain  as 
nearly  uniform  as  possible  for  different  individuals  and  for 
the  same  individual  at  different  times.  Accordingly,  some 
of  the,  most  important  experimental  results  in  this  field 
of  research  have  been  obtained  by  the  use,  of  practically 
meaningless  stimuli,  chiefly  stimuli  of  a  visual  kind,  o/. 
single  letters,  numbers,  or  senseless  syllables.  Thereby  we 
have  been  able  to  eliminate  associations  by  meaning,  to 
arrive  at  the  conditions  affecting  the  sheer  retentiveness  and 
reproducibility  of  a  presentation,  and  to  determine  the 
number  and  course  of  the  associations  which  are  formed 
among  the  members  of  a  series  of  such  objects.  It  is  true 
that  the  conditions  laid  down  may  depart  somewhat  widely 
from  those  which  obtain  in  daily  life.  But  only  from  such 
simple  beginnings  can  psychological  knowledge  advance 
beyond  that  st;ige  which  had  been  already  reached  before 
the  utilisation  of  experiment. 

The  Learning  and  fiaviny  Methods. — The  first  two 
methods  of  experiment  which  we  shall  describe  are  termed 
the  "learning"  and  the  "saving"  methods.  In  the  former, 
a  series  of  meaningless  syllables  is  read  through  at  a 
prescribed  uniform  speed,  and  the  readings  are  repeated 
until  the  first  correct  reproduction  can  be  effected.  Note  is 
taken  of  the  number  of  necessary  repetitions.  In  the  saving 
method,  a  varying  interval  is  allowed  to  elapse  after  the 


154  EXPERIMENTAL  PSYCHOLOGY 

task  has  been  learned  by  the  learning  method.  Then  the 
number  of  repetitions  is  ascertained  in  order  once  more  to 
effect  the  first  correct  reproduction.  It  is  compared  with 
the  number  of  previous  repetitions.  Throughout  all  these 
experiments  every  reading  is  followed  by  an  attempted 
reproduction,  until  a  successful  result  is  attained  (exp.  95). 

The  Prompting  Method. — In  the  "  prompting  "  method  a 
series  is  similarly,  but  imperfectly,  learnt,  and  the  accuracy 
of  reproduction  is  estimated  by  the  number  of  times  the 
subject  requires  to  be  prompted,  in  order  to  effect  a  perfect 
reproduction.  These  three  methods  have  been  applied  to 
the  learning  of  sensible  material  (prose  or  poetry),  as 
well  as  to  the  learning  of  senseless  material  (letters  or 
syllables). 

The  Scoring  Method.  —  The  "  scoring "  method  needs 
somewhat  complex  apparatus,  but  yields  on  the  whole  more 
precise  information  than  any  of  the  previous  three  methods, 
although  each  can  claim  special  merits.  In  this  method,  a 
series  of  syllables  is  read  a  prescribed  number  of  times, 
usually  in  trochaic  rhythm ;  that  is  to  say,  the  successive 
syllables  are  learnt  in  pairs,  the  first  member  of  each  pair 
being  strongly  accented  by  the  reader.  The  number  of 
repetitions,  however,  is  relatively  small,  and  is  always 
insufficient  for  a  perfect  learning.  An  interval  of  varying 
length  now  elapses ;  whereupon  the  accuracy  of  reproduction 
is  tested  by  re-exhibiting  in  various  order  the  first  members 
of  these  pairs,  the  subject  being  required  to  reproduce  the 
corresponding  second  members.  The  "  score  "  is  determined 
by  the  proportion  of  right  answers,  the  failures  by  the 
proportion  of  wrong  answers  or  of  no  answers  &t  all,  in  the 
entire  series.  Apparatus  may  be  introduced  in  this  scoring 
method,  which  permits  of  measurement  of  the  "  reproduction 
time,"  i.e.  the  interval  elapsing  between  the  recognition  of 
the  first  member  of  the  pair,  when  it  is  re-exhibited,  and  the 
reproduction  of  its  fellow.  The  reproduction  time  for 
effecting  a  score  is  called  the  "  scoring  time  "  (exp.  96). 


MEMORY  155 

[Comparison  of  the  Learning  and  Scoring  Methods. — In 
the  following  study  of  the  chief  results  obtained  by  experi- 
mental methods,  we  shall  have  to  refer  most  frequently  to 
the  learning,  saving,  and  scoring  methods.  In  each  of  these 
it  must  be  remembered  that  we  are  studying  the  repro- 
ducibility,  not  of  a  single,  but  of  numerous,  associated 
members  of  a  series ;  for  the  conditions  of  reproduction  are 
too  complex  to  permit  of  the  study  of  the  reproducibility  of 
individual  syllables.  In  the  learning  and  saving  methods, 
the  results  express  the  number  of  repetitions  just  necessary 
to  effect  perfect  repetition  of  a  series  of  syllables.  In  the 
scoring  method,  as  we  shall  immediately  see,  the  score  gives 
a  measure  of  the  mean  tendency  to  reproduction,  possessed 
by  the  first  syllables  of  the  pairs  in  the  series.  For,  when 
the  effects  of  perseverance  can  be  eliminated,  what  the 
scoring  method  tests  is  the  mean  tendency  of  the  syllables 
afc,e...t  to  reproduce  respectively  the  syllables  &,$,/..., 
with  which  they  have  become  associated. 
'jTl  In  the  learning  and  saving  methods,  the  series  is  treated 
as  a  whole,  and  the  data  are  complicated  by  the  formation  of 
associations  between  I  and  c,  d  and  et  etc.,  and,  as  we  shall  later 
see,  by  associations  between  more  distant  members.  Further 
complications  are  introduced  in  the  learning  and  saving 
methods  owing  to  the  fact  that  only  a  part  of  the  series  is 
reproduced,  so  long  as  the  learning  is  incomplete,  and  thus 
receives  more  frequent  repetition  than  the  rest ;  also  owing 
to  the  possibility  that  several  syllables  are  simultaneously 
in  the  field  of  vision  while  reading. 

The  fatigue,  which  is  involved  in  repeating  long  series  of 
syllables  by  the  learning  or  saving  method,  until  the  first 
correct  reproduction  is  effected,  becomes  much  reduced  in 
the  scoring  method,  where  reproduction  may  be  tested  after 
any  arbitrary  number  of  readings. 

The  results  obtained  by  the  learning  and  saving  methods 
are  liable  to  be  enormously  influenced  by  the  distribution  of 
individual  association  strengths.  The  accidental  presence  of 


156  EXPERIMENTAL  PSYCHOLOGY 

one  or  two  exceptionally  weak  associations,  in  a  series  learnt 
by  these  methods,  must  materially  increase  the  number  of 
repetitions  necessary  for  a  correct  reproduction ;  whereas, 
in  the  scoring  method,  the  score,  which  may  be  taken  at  any 
time,  is  independent  of  the  extent  to  which  certain  associa- 
tions may  lie  below  the  threshold,  and  merely  measures  the 
number  of  associations  which  are  effective  at  the  moment. 

Many  of  the  drawbacks  of  the  learning  and  saving 
methods,  to  which  we  have  drawn  attention,  are  in  some 
degree  avoidable  by  employing  a  "  modified  "  learning  and 
saving  method,  in  which,  as  in  the  scoring  method,  the 
syllables  are  successively  exposed  before  a  window,  are  read 
in  trochaic  rhythm,  and  are  learnt  in  pairs.  This  arrange- 
ment allows  us  experimentally  to  study  the  effect  of  replac- 
ing one  or  more  pairs  of  syllables  by  others.  By  learning 
series  which  are  altogether  new,  side  by  side  with  series, 
members  of  which  are  partly  old  and  partly  new,  the 
reliability  of  the  value  of  the  saving  time  may  often  be 
materially  increased.] 

Other  Methods. — Lastly,  there  are  two  methods  in  which 
recognition  can  be  experimentally  studied  apart  from  repro- 
duction. The  first  of  these  is  by  "selection,"  the  subject 
choosing  the  learnt  object  from  among  a  number  of  objects 
subsequently  exhibited.  The  second  is  by  the  use  of 
"  identical  series."  Here  the  entire  series,  which  has  been 
imperfectly  learnt,  is  re-exhibited,  and  the  subject,  wholly 
ignorant  of  this  procedure,  is  asked  whether  a  change  has 
or  has  not  been  effected. 

Relation  between  Scoring  Time  and  Association  Strength. 
— By  means  of  the  scoring  method,  it  can  be  shown  that 
those  pairs  of  syllables,  the  members  of  which  possess  the 
most  lasting  (or  strongest)  associations,  also  give  the  quickest 
scoring  times.  A  given  number  of  series  of  syllables  is  read 
until  they  have  been  learnt  imperfectly.  Each  series  is 
twice  tested,  first  twelve  minutes  and  again  twenty-four 
hours  after  the  last  reading.  The  scoring  times  are  classed 


MEMORY  157 

acccording  as  their  values  are  greater  or  less  than  200(K 
It  is  found  that  those  pairs  of  syllables,  which  at  the  earlier 
reproduction  had  given  scoring  times  less  than  2000°",  yield 
66  per  cent,  of  the  scores  at  the  later  reproduction,  while 
those  pairs,  which  at  the  earlier  reproduction  had  given 
scoring  times  greater  than  2000°",  yield  only  32  per  cent, 
of  the  scores  at  the  later  reproduction.  Thus  there  appears 
to  be  a  correlation  between  the  reproduction  times  and  the 
durability  of  associations,  and  hence  presumably  between 
the  former  and  their  strength. 

An  increase  in  average  scoring  time  need  not  in  all 
circumstances  imply  a  decline  in  general  reproductive 
tendency.  For,  as  with  successive  readings  fresh  individual 
associations  in  the  series  rise  above  the  threshold,  they 
naturally  tend  to  give  longer  scoring  times  than  the  easier 
and  previously  effective  associations,  and  thus  may  raise 
the  mean  scoring  time. 

Further,  under  certain  conditions  the  reproductive 
tendency  may  increase,  in  spite  of  an  unchanged  average 
scoring  time.  When,  for  example,  an  increasing  number 
of  repetitions  ultimately  fails  to  shorten  the  scoring  times 
beyond  a  certain  limit,  it  would  be  rash  to  conclude  that 
further  repetitions  produce  no  change  in  the  reproductive 
tendency  or  durability  of  associations. 

Influence  of  the  Length  of  the  Series. — The  number  of 
individual  words  or  syllables,  which  can  be  perfectly  learnt 
after  one  reading  by  the  learning  method,  is  found  to 
depend  on  their  nature.  One  investigator  reports  that 
after  a  single  reading  he  can  immediately  reproduce  seven 
disconnected  words,  or  eighteen  words  of  a  poem,  or  twenty- 
two  prose  words ;  another  finds  that  he  can  learn  in  a  single 
reading  eleven  figures,  or  nine  disconnected  words,  or  seven 
letters  or  senseless  syllables. 

The  influence  of  the  number  of  senseless  three-letter 
syllables  on  the  number  of  repetitions  needed  for  immediate 
reproduction  by  the  learning  method,  has  been  investigated 


158  EXPERIMENTAL  PSYCHOLOGY 

in  the  following  way.  At  each  sitting,  nine  series  of  twelve 
syllables,  or  six  series  of  sixteen  syllables,  or  three  series 
of  twenty-four  syllables,  or  two  series  of  thirty-six  syllables, 
are  read  at  a  uniform  speed,  each  series  being  repeated 
until  the  first  correct  reproduction  is  effected.  The  follow- 
ing is  the  average  of  results  obtained  after  a  considerable 
number  of  such  sittings  : — 

Number  of  syllables  in  series        .        .7      12      16      24      36 
Number  of  repetitions  needed       .         .1       16*6   30      44      55 

The  strength  of  the  associations,  which  are  effected 
between  individual  members,  by  learning  such  a  series  of 
syllables,  must  increase  with  successive  repetitions.  After 
the  first  reading,  very  few  associations  have  the  requisite 
strength  or  force  to  be  effective.  With  subsequent  readings, 
first  some,  later  others,  rise  above  the  threshold  and  thus 
become  effective.  Finally,  a  certain  mean  strength  of  all 
associations  is  reached,  which  first  allows  of  the  first  correct 
reproduction.  The  greater  the  number  of  syllables  in  the 
series,  the  greater  must  be  the  fatigue  and  the  less  concen- 
trated the  attention,  in  later  repetitions  and  reproductions. 
Hence,  in  the  learning  method,  the  mean  association  strength 
of  the  members  of  a  long  series  must  be  greater  than  that 
of  the  members  of  a  short  series,  in  order  just  to  effect  a 
correct  reproduction. 

[Let  us  now  turn  to  a  somewhat  similar  investigation 
conducted  by  the  scoring  method.  In  a  set  of  experiments 
lasting  twenty-four  days,  two  series  each  of  twelve  syllables 
and  two  each  of  eighteen  syllables  are  read  a  certain 
number  of  times  daily.  The  series  read  are  entirely  new 
each  day.  The  number  of  repetitions  of  each  series  is  for 
some  series  small  (seven,  eight,  or  nine),  for  others  large 
(twelve,  thirteen,  or  fourteen).  Care  is  taken  that  the 
subject  is  unaware  how  many  times  the  presented  series 
will  be  repeated  before  reproduction  is  required.  The  posi- 
tion of  the  long  and  short  series  relatively  to  one  another  is 


MEMORY 


159 


changed  daily.  Five  minutes  elapse  between  the  last  read- 
ing and  the  re-exhibition  of  the  syllables  for  reproduction. 
The  results  are  as  follows,  r  representing  the  number  of 
scores  in  ratio  to  unity,  Tr<2000cr  being  the  absolute 
number  of  those  scoring  times  which  are  less  than  two 
seconds : — 


Twelve-syllable  series 
Eighteen-syllable  series  . 

Few  Repetitions. 

Many  Repetitions. 

r     ZV<2000<r. 

r      2V<200(K 

0-60           33 
0-47           21 

071           40 
0-69           27 

Clearly  the  more  frequent  readings  produce  a  very  much 
greater  effect  on  the  longer  than  on  the  shorter  series.  In 
other  words,  the  number  of  associations,  which  after  a  few 
repetitions  are  almost  ready  to  rise  above  the  threshold  and 
to  become  effective,  is  considerably  greater  in  longer  than  in 
shorter  series.] 

[Influence  of  the  Position  of  Syllables,  and  of  Accent 
and  Rhythm. — When  a  series  of  ten  or  twelve  members  is 
learnt  by  the  prompting  method  (page  154),  it  is  seen  that 
the  impression  made  by  the  different  members  varies 
according  to  their  position  in  the  series.  The  following 
experimental  data  indicate  that  the  first  member  of  the 
series  is  most  easily  remembered,  that  the  second  and  last 
members  follow  next : — 


Order  of  Word  in  Series. 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

Number  \  in  12-word  series 

0 

11 

21 

134 

35 

36 

36 

29* 

43 

37i 

34 

11 

of 
'  prompts  'J  in  10-word  series 

0 

3 

6 

9 

23 

24 

31* 

25 

23 

54 

... 

... 

160  EXPERIMENTAL  PSYCHOLOGY 

The  learning  of  such  a  series  is  therefore  comparable  to 
the  building  of  a  bridge,  the  ends  of  which  are  begun  first, 
the  builder  working  towards  the  middle. 

That  the  position  of  syllables  within  a  series  and  their 
accent  have  a  most  important  influence  on  learning,  is  shown 
by  the  following  experiment.  Pairs  of  twelve  syllable- 
series,  each  series  having  previously  been  learnt  in  trochaic 
rhythm,  according  to  the  modified  learning  method  (page 
156),  are  intermixed,  rearranged,  and  subsequently  relearnt. 
Let  us  designate  a  given  pair  of  series : — Ix  I2 13  .  .  .  I12, 
IIj  II2  II3  .  .  .  II12.  Different  pairs  are  now  arranged  in 
different  ways.  In  some  the  original  accent  is  retained, 
while  the  order  of  the  syllables  is  changed,  e.g.  Ix  II10,  InII8, 
I9  II6,  I7  II4,  I5  II2,  I3  II12,  or,  by  a  different  plan,  Iu  II12, 
I9  II10,  I7  II8,  I5  II6,  I3  II4,  Ix  II2.  In  others  the  original 
pairs  (save  the  first)  and  the  original  accents  are  main- 
tained thus — Iu  II12,  I9  I10,  II7 II8,  I5  I6,  II3  II4,  Ix  I2.  In 
three  other  pairs  these  orders  are  reversed,  so  that  the 
originally  accented  syllables  now  become  unaccented,  and 
vice  versd,  e.g.  I2  IIn,  I12  II9,  I10  II7,  I8  II6,  I6  II3,  I4  IIr 
It  is  found  that,  when  these  various  derived  series  are  re- 
learnt  twenty-four  hours  after  the  learning  of  the  original 
series,  by  far  the  greatest  saving  is  effected  by  that  arrange- 
ment in  which  the  original  pairs  and  accents  are  preserved. 

The  scoring  method  shows  clearly  that  associations  are 
formed  by  virtue  of  position.  Not  infrequently  when  a 
wrong  syllable  is  given,  it  is  found  to  have  occurred  in  a 
corresponding  place  in  a  series  which  had  been  learnt  some 
time  before.  There  appear  to  be  wide  individual  differ- 
ences in  the  liability  of  subjects  to  associations  of  this 
kind. 

Most  subjects  find  it  difficult,  if  not  impossible,  to  avoid 
rhythmisation,  when  they  are  confronted  with  the  task  of 
learning  a  series  of  senseless  syllables.  They  tend  spon- 
taneously to  develop  rhythmic  movements  of  the  head, 
trunk,  or  limbs,  and  they  find  that  a  favourable  rhythm,  just 


MEMORY  161 

as  a  favourable  speed  of   reading,  exists  for   them,  which 
proves  the  most  economical  method  of  learning.] 


BIBLIOGRAPHY. 

T.  Fechner,  Elemente  d.  PsychophysiJc,  Leipzig,  1860,  ii.  468.  H. 
Ebbinghaus,  Uelerdas  Geddchtniss,  Leipzig,  1885.  H.  K.  Wolfe,  "Untersuch. 
iiber  d.  Tongedachtniss, "  Philosoph.  Stud.,  1886,  iii.  534.  H.  Miinsterberg, 
Beitrage  zur  experimentelle  PsyclioL,  1889-92,  Hft.1-4.  G.  E.  Miiller  and  F. 
Schumann,  ' '  Experimentelle  Beitrage  zur  Untersuchung  des  Gedachtnisses," 
Ztsch.  f.  Psychol  u.  Physiol.  d.  Sinnesorgane,  1894,  vi.  81,  257,  G. 
Aschaffenburg,  "Experimentelle  Studien  iiber  Associationen,"  Psychol. 
Arbeiten,  1895,  i.  209  ;  1897,  ii,  1.  J.  Colin,  "Experimentelle  Untersuch- 
ungen  iiber  d.  Zusammenwirken  d.  akustisch-motorisclien  u.  d.  visuellen 
Gedachtnisses,"  Ztsch.  f.  Psychol.  u.  Physiol.  d.  Sinnesorgane,  1897,  xv.  161. 
G.  E.  Miiller  and  A.  Pilzecker,  "Experimentelle  Beitrage  zur  Lehre  vom 
Gedachtniss,"  ibid.,  Ergiinzungsbd.  i.  1900.  G.  M.  Whipple,  "An  Analytic 
Study  of  the  Memory  Image  and  the  Process  of  Judgment  in  the  Discrimina- 
tion of  Clangs  and  Tones,"  Amer.  Journ.  of  Psychol.,  1901-2,  xii,  xiii.  A. 
Binet,  L'£tude  exp6rimentale  de  V Intelligence,  Paris,  1903.  E.  Claparede, 
IS  Association  des  Idees,  Paris,  1903.  H.  J.  Watt,  "Experimentelle  Beitrage 
zu  einer  Theorie  d.  Denkens,"  Arch.f.  d.  ges.  Psychol.,  1905,  iv.  289. 


II 


CHAPTEE  XIII1 
ON   MEMORY  (concluded) 

The  Rate  of  Forgetting. — The  saving  method  affords  a 
means  of  studying  this  subject.  The  following  is  a  record 
of  163  experiments,  nearly  all  of  which  consist  in  learning 
eight  thirteen-sy liable  series  and  in  re-learning  them  at  a 
prescribed  rate  of  reading  after  a  varying  interval,  until  two 
consecutive  perfect  reproductions  are  effected ;  the  economy 
of  time,  spent  in  re-learning,  being  in  each  instance  noted. 


Re-learning  after 
x  hours. 

Percentage  of  Time 
Saved. 

Percentage  of  Time 
Lost. 

X  — 

0-3 

58-2 

41-8 

1-0 

44-2 

55-8 

8-8 

35'8 

64-2 

24 

337 

66-3 

2x24 

27-8 

72-2 

6x24 

25-4 

74-6 

31x24 

21-1 

78-9 

These  experiments  are  admittedly  very  crude,  and  it 
would  be  rash  to  infer  from  them  that  one-half  of  the 
matter  is  forgotten  after  the  first  half-hour,  two-thirds  in 


1  See  footnote  to  Chapter  III. 

162 


MEMORY  163 

nine  hours,  three-quarters  after  six  days,  or  four-fifths  after 
a  month.  Yet  there  can  be  no  doubt  that  in  the  process 
of  forgetting  a  very  great  initial  fall  of  memory  occurs, 
followed  by  losses  which  become  increasingly  less. 

*  That  the  most  rapid  fall  of  memory  occurs  immediately 
after  learning  is  also  shown  by  the  fact  that,  when  associa- 
tions which  have  just  been  formed  are  compared  with 
others  ten  minutes  old,  there  is  a  far  greater  difference  in 
the  number  of  scores  than  when  associations  ten  minutes 
old  are  compared  with  others  which  are  twenty-four  hours 
old.  Now  the  tendency  towards  reproduction  in  the  scoring 
method  is  the  outcome  of  perseverance  as  well  as  of 
association  strength.  It  is  perhaps  mainly  because  of  the 
rapid  falling  off  of  perseverance  (which  is  most  effective  in 
immediate  memory)  that  the  youngest  associations  suffer 
more  at  the  hands  of  time  than  older  ones. 

Retro-active  Inhibition. — On  the  other  hand,  time  has  a 
favourable  effect  on  associations,  in  that  it  allows  of  their 
consolidation.  There  is  reason  to  believe  that  the  act  of  form- 
ing one  association  c-d,  just  after  the  formation  of  another 
association  a-b,  inhibits  the  latter.  ( This  "  retroactive 
inhibition  "  is  yet  another  cause  of  the  greater  difficulty  in 
learning  longer  than  shorter  series.)  It  disappears  in  course 
of  time,  whereupon  the  various  associations  are  better  con- 
solidated. The  familiar  practice  of  imperfectly  learning  a 
task  at  night  and  allowing  rest  to  improve  the  associations  by 
the  morning,  depends  partly  on  this  process  of  consolidation. 
The  experimental  investigation  of  retro-active  inhibition 
has  shown  that,  when  the  learning  of  a  series  of  syllables 
precedes  by  a  few  seconds  the  readings  of  a  second  series, 
the  subsequent  number  of  scores  and  of  short  scoring  times 
of  the  first  series  is  smaller  than  when  some  minutes  elapse 
between  the  readings  of  the  two  series. 

Such  retro-active  inhibition  may  be  likewise  demon- 
strated, when  the  interval  between  the  last  reading  and  the 
reproduction  is  only  one  or  two  hours,  or  even  when  it  is 


1 64  EXPERIMENTAL  PSYCHOLOGY 

twenty-four  hours.  There  is  also  evidence  of  its  action,  not 
only  when  the  second  task  is  similar  to  the  first,  but  also 
when  it  is  of  a  quite  different  nature.  Thus,  when  four 
series  of  twelve  syllables,  which  we  may  call  A,  are  read 
eight  times,  and  are  followed  by  so  close  an  examination  of 
three  pictures  as  to  enable  the  subject  to  pass  a  minute 
examination  on  their  contents  ;  and  when  four  like  series  of 
syllables,  B,  are  similarly  read,  the  like  interval  however 
between  reading  and  reproduction  now  being  as  restful  as 
possible ;  the  score  and  the  average  scoring  time  are  found  to 
be  0-24  and  2950°-  in  series  A  and  0*56  and  2490°-  in  series  B. 

Retro-active  and  Remote  Association. — So  far  we  have 
chiefly  dealt  with  "  principal^^LSsociations, — the-  associations 
formed  between  the  immediately  consecutive  members  of  a 
series  in  a  forward  direction.  We  have  now  to  consider  the 
evidence  in  favour  of  the  existence  of  certain  "  subsidiary  " 
associations,  more  particularly  of  associations  formed  in  the  re- 
verse direction,  and  of  forward  associations  formed  between  not 
immediately  consecutive  members  of  a  series.  These  we  shall 
respectively  call  "  retro-active  "  and  "  remote  "  associations. 

Six  series,  each  of  sixteen  syllables,  are  learnt,  which  we 
may  designate  as  Ix  12 13  .  .  .  116,  IIX  II2 II3  .  .  .  II16,  IIIj 
III2  .  .  .  III16,  IV,  ...  I Vjfll  vi  ;  -  •  V16,  and  VIX  .  .  .  VI16. 
Twenty-four  hours  later,  six  derived  series  are  learnt,  and 
the  saving  of  time  is  noted  by  the  saving  method.  These 
derived  series  are  prepared  from  the  above  in  one  of  five 
different  methods,  a,  b,  c,  d,  and  e ;  but  on  any  one  day  all 
six  of  the  derived  series  are  prepared  by  the  same  method. 

In  method  a,  alternate  syllables  are  omitted,  so  that  the 
six  series  run  : — 

(i.)  i,  i, . . .  i15 1,  i4 . . .  iw 

(ii.)  n,  n3 . . .  n16  n2  n4 . . .  n16 


(vi.)  VIX  VI3  .  .  .  VI16  VI2  VI4  .  .  .  VI16 


MEMORY 


165 


In  method  I,  two  consecutive  syllables  are  removed,  so 
that  the  first  of  the  six  series  runs  :  — 

Ix  I4  I7  I10  I13  I16  I2  I5  I8  In  I14  I3  I6  I9  I12  I15 

In  method  c,  three  consecutive  syllables  are  removed. 
In  method  d,  seven  consecutive  syllables  are  removed,  the 
first  two  series  running  thus  :  —  • 

(i.)  I,  I,  II,  II,  III,  III,  IV,  IV,  V,  V9  VIX  VI9  I2  I10 

n,  nw 
(ii.)  in,  mw  iv2  iv10  v2  v10  vi,  vi10  13  in  ii,  n 


w 

III3  !!!„  IV,  IVU 


n 


In  method  e,  the  position  of  the  first  and  last  members 
of  each  of  the  original  six  series  is  alone  retained,  the  other 
fourteen  members  of  each  series  being  arranged  quite  in 
haphazard  order. 

The  following  results  have  been  obtained  :  — 


Method. 

Learning  Time 
of 
Original  Series. 

Learning  Time 
of 
Derived  Series 
24  Hours  later. 

Percentage 
Saved. 

a 

1275" 

1138" 

10-8 

b 

1260" 

1171" 

7-0 

c 

1260" 

1186" 

5-8 

d 

1268" 

1227" 

3-3 

e 

1261" 

1255" 

0'5 

Method  e  was  expressly  devised  to  demonstrate  that  the 
saving  is  not  due  to  familiarity  with  the  syllables  of  the 
original  series. 

From  these  data  the  conclusion  has  been  drawn  that,  in 
learning  a  series  of  presentations,  associations  are  formed 
not  only  between  immediately  consecutive  members,  but 
also  between  those  which  are  not  immediately  consecutive, 


1 66  EXPERIMENTAL  PSYCHOLOGY 

the  strength  of  this  remote  association  diminishing  with  the 
distance  of  the  members  from  one  another. 

Similar  experiments  have  been  conducted  with  the 
object  of  proving  retro-active,  i.e.  backward  association.  It 
has  been  found  that  while  33*3  per  cent,  is  saved  when  a 
sixteen-syllable  series  is  relearnt  in  the  same  order  twenty- 
four  hours  later,  there  is  still  a  distinct  saving,  namely,  one  of 
12  per  cent.,  when  another  such  series  is  relearnt  after  a  like 
interval  in  completely  reversed  order,  I16  I15  I14  .  .  .  I15  and 
that  there  is  even  a  saving  of  5  per  cent.  wrhen  the  derived 
order  is  obtained  both  by  reversal  and  by  the  omission  of 
alternate  syllables,  I16  I14  I12  .  .  .  I2  !„  I13  .  .  .  Ir 

In  these  experiments  the  possibility  of  simultaneous 
vision  of  more  than  two  consecutive  syllables  during  read- 
ing was  not  adequately  safeguarded  as  it  is  in  the  scoring 
and  in  the  modified  saving  methods.  The  results  are 
further  complicated  by  the  undoubted  tendency  of  any 
given  syllable  in  the  derived  series  to  reproduce  the  con- 
secutive syllable  of  the  original  series ;  by  virtue  of  which 
that  consecutive  syllable  would  continue  to  be  held  in  a 
certain  "readiness,"  and  would  in  consequence  be  more 
easily  learnt,  the  earlier  it  actually  appeared  in  the  derived 
series.  Yet  a  closer  examination  of  such  complications 
and  the  results  of  subsequent  more  laborious  experiments, 
in  which  these  objections  have  with  some  success  been 
eliminated,  leave  room  for  no  doubt  as  to  the  formation  of 
retro-active  and  remote  associations. 

The  Behaviour  of  Related  Associations. — We  have  just 
alluded  to  the  tendency  of  a  syllable  to  be  held  in  "  readi- 
ness," when  another  syllable,  with  which  it  has  been  pre- 
viously associated,  is  subsequently  learnt  in  association  with 
a  different  syllable.  This  tendency  deserves  to  be  examined 
more  closely;  it  has  been  also  termed  the  "strengthening 
of  related  associations."  When  a  syllable  a,  which  has  been 
already  firmly  associated  with  a  syllable  6,  is  presented 
with  cy  the  association  a-b  is  strengthened,  b  either  being 


MEMORY  167 

actually  recalled,  or  merely  being  held  in  readiness,  so  that 
subsequent  learning  of  a-b,  as  evidenced  by  the  number  of 
scores  and  the  brevity  of  scoring  times,  is  facilitated  by  the 
act  of  learning  a-c. 

[This  concomitant  increase  in  related  association  strength 
has  been  found  to  vary  in  different  individuals.  It  is 
most  manifest  when  the  association  a-b  is  already  of  fair 
strength,  before  a  and  c  are  presented,  and  it  differs 
therein  from  the  action  of  an  increased  number  of  repe- 
titions, which  improve  the  weaker  at  least  as  much  as  the 
stronger  associations.  Its  effect  is  also  dependent  on  other 
factors,  e.g.  the  shortness  of  the  interval  elapsing  between 
the  last  reading  of  a-c  and  the  testing  of  the  improvement 
of  the  association  a-b.  But  into  these  and  other  con- 
ditions we  cannot  enter  here.  When,  on  the  other  hand, 
the  association  a-b  has  been  but  imperfectly  formed,  or 
more  especially  when  it  has  not  been  formed  at  all,  the 
formation  of  an  association  a-c,  so  far  from  being  favour- 
able, is  actually  antagonistic  to  the  subsequent  formation  of 
the  association  a-b.  That  is  to  say,  while  the  readings  of 
a-c  facilitate  the  subsequent  reconstruction  of  the  previously 
well-formed  association  a-b,  they  inhibit  the  subsequent 
formation  of  the  association  a-b,  if  it  be  practically  or 
altogether  non-existent  at  the  time  of  the  formation  of  a-c.] 

[Unconscious  Association. — In  connection  with  the  con- 
comitant strengthening  of  related  associations,  the  following 
experiment  is  of  considerable  interest.  Six  series  of  six- 
teen syllables,  Ix  .  .  .  I16,  IIj  .  .  .  II16,  etc.,  are  repeated  a 
definite  number  of  times.  Twenty-four  hours  later,  six 
derived  series  are  learnt  in  the  following  order : — 

(i.)  ix  i3 1. . . .  i15  iij  n3  ii. . . .  n15 
(u.)  i2  i4  i6  .  .  .  i16  n2  n4  n6 . . .  n,8 

(iii.)  Ill,  .  .  .  III]5  IV,  ...  IV16 
(iv.)  III2  .  .  .  III16 IV2  .  .  .  IV16 
(v.)  V,  ...  V15  VIX  .  .  .  VI15 
(vi.)  T.  .  .  .  V16  VI2  .  .  .  VI16 


1 68  EXPERIMENTAL  PSYCHOLOGY 

It  is  found  that  the  second,   fourth,  and   sixth  series   are 
learnt  more  easily  than  the  first,  third,  and  fifth  series. 

Two  explanations  of  this  result  are  possible ;  and  one  is 
undoubtedly  true,  namely,  that  the  learning  of  I:  I3  I5,  etc.,  in 
(i.)  brings  into  readiness  the  syllables  I2  I4  I6,  etc.,  and  so 
facilitates  the  subsequent  learning  of  (ii.).  But  there  may 
be  a  further  explanation.  It  is  conceivable  that  the  suc- 
cessive bringing  of  the  syllables  I2  I4,  etc.,  into  readiness 
unconsciously  lays  down  associations  or  reinforces  the 
previous  remote  associations  between  them.  This  possi- 
bility has  suggested  very  long  and  laborious  experiments, 
the  object  of  which  has  been  to  determine  whether  the  suc- 
cessive bringing  of  syllables  into  readiness  causes  them  to 
become  unconsciously  associated  with  one  another.  The 
evidence  is  by  no  means  decisive,  but  it  is  perhaps  in  favour 
of  the  unconscious  formation  of  such  associations.] 

Initial  and  Group  Reproduction. — If  syllables  a,  b,  c, 
etc.,  be  learnt  in  groups  of  three  in  anapaestic  measure 
(v^w-),  a  tends  to  be  reproduced  more  frequently  than  b, 
when  the  accented  member  c  is  subsequently  presented. 
There  exists  a  tendency  to  "  initial  reproduction."  So,  too, 
when  a  series  has  been  learnt  in  trochaic  measure  (-  ^  ),  the 
preceding  accented  member  is  reproduced  much  more  often 
than  the  following  accented  member,  when  any  second  or 
unaccented  member  is  presented.  In  this  case  the  tendency 
to  initial  reproduction  is  perhaps  complicated  by  the  un- 
doubted tendency  of  any  member  of  a  group  to  revive  the 
whole  group.  Our  experience  in  daily  life  familiarises  us 
with  this  latter  tendency,  the  word  "  gables,"  for  instance, 
reviving  the  whole  phrase,  "  the  house  with  the  seven 
gables." 

[The  Independence  of  Subsidiary  and  Principal  Associa- 
tions.— Such  retro-active  associations  are  more  evident  in 
recently  than  in  earlier  learnt  series.  By  the  scoring 
method  it  has  been  found  that;,  when  five  minutes  elapse 
between  the  last  reading  and  reproduction,  of  forty-two 


MEMORY  169 

wrong  answers  there  are  seven  in  which  the  last  syllable  of 
the  previous  pair,  instead  of  the  immediately  following 
syllable  of  the  same  pair,  is  reproduced  ;  whereas  there  are 
no  such  wrong  answers  in  other  learnt  series,  when  twenty- 
four  hours  are  allowed  to  elapse.  Even  if  the  number  of 
readings  in  the  two  experiments  be  so  arranged  that  the 
scores  (i.e.  the  strengths  of  the  principal  associations)  in 
one  series  are  the  same  five  minutes  after  learning  as 
they  are  in  another  series  twenty-four  hours  after  learning, 
the  retro -active  associations  of  the  latter  are  always  fewer. 
In  other  words,  retro-active  associations  wane  more  rapidly 
than  principal  associations. 

A  similar  want  of  correlation  is  found  between  the 
strengths  of  remote  and  principal  associations.  Increased 
repetition  is  found  to  have  a  much  less  improving  effect  on 
remote  than  on  principal  associations.] 

[Mediate  Association. — While  there  is  good  reason  for 
believing  in  the  existence  of  remote  association,  the  evidence 
in  favour  of  what  is  called  "mediate"  association  is  un- 
questionably very  weak.  Mediate  association  occurs  if,  when 
a  is  associated  with  b,  and  1}  later  with  c,  the  subsequent 
presentation  of  a  yields  c  without  the  reproduction  of 
b.  Eemote  association,  on  the  other  hand,  is  merely  an 
example  of  association  by  temporal  or  spatial  contiguity. 
In  the  series  a,  b,  c,  c  follows  on  a,  forms  part  of  a,  b,  c,  and 
is  therefore  associated  with  a.  In  mediate  association,  the 
presentations  do  not  occur  in  this  way.  The  association 
between  a  and  b,  and  that  between  b  and  c,  are  separately 
formed  at  different  times;  a,  b,  and  c  do  not  belong  to  a 
single  series. 

The  first  experimental  inquiry  into  the  existence  of 
mediate  association  was  conducted  in  a  somewhat  crude  and 
unconvincing  fashion.  Three  series  of  words  were  prepared, 
namely  (a)  German  words ;  (b)  their  equivalents  in  Japanese 
characters ;  and  (c)  the  same  Japanese  words  written  in 
Eoman  characters.  For  brevity's  sake  let  us  call  these 


170  EXPERIMENTAL  PSYCHOLOGY 

three  groups  of  presentations  a,  b,  and  c.  Cards  were 
exposed  in  irregular  order  before  the  subject,  first  those 
containing  any  b  beside  its  corresponding  c,  and  next  others, 
in  equal  number,  showing  any  a  beside  its  I.  Thereupon, 
the  subject  was  asked  if  he  had  observed  any  relation 
between  the  a-b  cards  and  the  b-c  cards,  and  the  experiment 
was  continued  only  if  he  declared  that  he  had  not  observed 
any  relation.  Then  cards,  eacli  bearing  any  one  of  the  a 
words  only,  were  exhibited,  and  the  subject  was  asked  to 
state  the  first  word  which  occurred  to  him.  There  were 
said  to  be  many  instances  in  which  he  returned  the  appro- 
priate c,  without  the  Japanese  character  b  ever  having 
occurred  to  him. 

Later  investigators,  however,  have  failed  to  find  similar 
evidence.  It  is  true  that  in  reaction  experiments  upon 
simple  association,  reaction  words  and  introspective  data 
have  been  obtained,  which  seem  to  show  the  occurrence  of 
mediate  association.  But  very  many  of  these  replies  are 
capable  of  quite  a  different  interpretation.  When,  for 
example,  the  exhibition  of  the  word  house  yields  the 
reply  marines,  the  answer  may  be  due  merely  to  the 
confusion,  through  similarity,  of  house  and  horse.  We 
need  not  suppose  that,  the  word  horse  being  associated 
by  verbal  similarity  with  the  word  house,  and  by  verbal 
contiguity  with  the  word  marines,  we  have  a  case  of 
mediate  association  between  house  and  marines. 

Such  confusion  by  similiarity  is  of  frequent  occurrence, 
alike  in  ordinary  life  (e.g.  when  a  child  calls  an  animal  by 
the  wrong  name,  or  when  a  person  makes  a  slip  in  talking), 
and  in  the  experimental  investigation  of  memory.  If,  for 
example,  A-B  is  learnt,  and  if  a  resembling  A  is  later 
presented,  B  is  liable  to  be  produced.  This  has  been 
called  "  wrong  association  by  active  substitution."  Another 
variety  of  wrong  association,  which  also  comes  into  play 
in  the  just  mentioned  association  experiments,  occurs 
when  A-B  is  learnt  but  A-b  is  reproduced,  b  having 


MEMORY  17 1 

some  resemblance  to  B.  This  has  been  called  "  wrong 
association  by  passive  substitution."  These  associations  by 
substitution  are  capable  of  explaining  many  of  the  replies 
which  at  first  sight  appear  to  favour  the  play  of  mediate 
association. 

The  fact  that  the  subject  is  not  conscious  of  the  presence 
of  the  medial  connecting  link,  until  he  has  given  the  reaction 
word,  affords  no  proof  that  the  association  of  the  former 
with  each  of  the  words  is  physiologically  inactive.  In  daily 
life,  a  long  time  may  elapse  before  the  connecting  link 
between  two  successive  thoughts  is  apparent.  Nay,  the 
attention  may  be  so  fixed  upon  the  initial  or  final  member 
of  the  trio,  that  the  intermediate  member  fails  altogether 
to  effect  consciousness.  This  way  of  stating  the  process, 
however,  is  very  different  from  the  assertion  that  a  direct 
mediate  association  is  formed  between  the  first  and  the 
third.] 

The  Distribution  of  Repetitions. — It  is  a  familiar  ex- 
perience that  a  lesson  is  better  retained  when  the  learning 
extends  over  a  considerable  period  of  time,  than  when  the 
task  is  learnt  by  the  same  number  of  repetitions  at  a  single 
sitting.  The  inferior  value  of  accumulated,  as  compared 
with  distributed,  repetitions  is  at  first  sight  attributable 
to  differences  in  fatigue,  interest,  or  attention.  But  the 
following  experiment  proves  that  such  an  explanation  is 
inadequate. 

Thirty-six  series  of  twelve  syllables  are  learnt  in  trochaic 
rhythm  by  twenty-four  repetitions  distributed  in  three 
different  ways,  namely,  over  (a)  three,  (6)  six,  and  (c)  twelve 
days.  Thus  the  series  which  we  shall  call  a  are  repeated 
eight  times  on  three  consecutive  days,  the  series  &  four  times 
on  six  consecutive  days,  series  c  twice  on  twelve  consecutive 
days.  On  the  fourth,  seventh,  and  thirteenth  days,  a,  I,  and 
c  are  respectively  tested  by  the  scoring  method.  The 
research  extends  over  two  periods  each  of  fourteen  days ; 
in  each  such  period  six  of  series  a,  six  of  6,  and  six  of  c  are 


c5     I, 


172  EXPERIMENTAL  PSYCHOLOGY 

learnt.     The  eighteen  series,  a^-a^  \-b^  ^-Cg,  are  learnt  in 
the  following  order  on  different  days  : — 

First  day 

«/  *JL4iX4P«1K«V« 

Second          „         &x     c2     al     c3     b2     c4     a2     c5     b3     ce 
Third  „         c2     ax     c3     \     c4     a2     c5     &3     c6     ^ 

Fourth 

Seventh        „ 
Eighth 

Twelfth 
Thirteenth    „ 
Fourteenth  „ 

The  series  are  enclosed  in  brackets  on  the  days  when  they 
are  tested.  As  the  number  of  series  tested  on  the  thirteenth 
day  is  unusually  heavy,  this  day's  task  is  lightened  by 
inserting  two  series  which  have  been  already  tested ;  this  is 
also  done  on  the  last  day. 

A  pause  of  about  two  minutes  separates  the  reading  of 
consecutive  series.  Each  day's  experiment  lasts  about 
thirty-five  minutes.  During  the  whole  research  seventy- 
two  pairs  of  syllables  are  learnt  in  trochaic  rhythm  for  each 
of  the  series  a,  5,  c.  The  following  are  the  results  given  by 
two  individuals,  X  and  Y,  rl  representing  the  absolute 
number  of  scores,  Tr  the  average  scoring  time  : — 


a 

b 

0 

ri 

Tr 

r1 

Tr 

ri 

Tr 

X 

18 

2496°" 

39 

2213°" 

53 

2007°" 

Y 

7 

2429°" 

31 

1570°" 

55 

1675°" 

These  data  clearly  show  that  distributed  readings  yield 


MEMORY  173 

a  greater  number  of  scores  than  accumulated  readings. 
They  also  show  that  the  cause  cannot  lie  in  differences  of 
fatigue  or  of  interest,  since  the  effect  is  visible  both  in  b  and 
in  c,  which  only  differ  by  two  readings  daily.  That  the 
average  scoring  times  do  not  show  a  proportionate  increase 
in  speed  is  due  to  the  overweighting  influence,  in  I  and  c,  of 
associations  which  are  only  just  above  the  threshold,  and 
consequently  have  relatively  long  scoring  times  (page  157). 

Nor  is  the  superior  retentivity  of  the  most  distributed 
readings  due  to  the  involuntary  revival  of  the  syllables  by 
the  subject  between  the  periods  of  reading.  He  was  carefully 
instructed  not  to  think  of  the  syllables.  Besides,  the  same 
results  have  been  obtained  in  other  experiments,  where  the 
accumulated  repetitions  are  compared  with  groups  of  repeti- 
tions which  are  separated  by  minutes  instead  of  by  days 
from  one  another. 

The  Influence  of  the  Age  of  Association  on  the  Results  of 
^Repetition. — There  are  probably  several  factors  contributing 
to  produce  this  result,  but  evidence  points  very  strongly  to 
the  influence  of  the  following.  ^When  two  associations  are 
of  like  strength,  but  of  unlike  age,  repetition  increases  the 
strength  of  the  older  more  than  that  of  the  younger  associa- 
tion.] The  importance  of  this  factor  is  clearly  shown  in  the 
following  experiment. 

Two  series  of  twelve  syllables  (series  Av  A\)  are  each 
read  thirty  times.  Twenty-four  hours  later  they  are  tested, 
one  by  the  saving,  the  other  by  the  scoring  method.  There- 
upon two  new  series  of  twelve  syllables  (series  Bv  £\)  are 
immediately  read  four  times,  and  are  tested,  one  by  the 
saving,  the  other  by  the  scoring  method,  one  minute  after 
the  last  reading  of  each.  Then  two  more  series  (A2,  A'2)  are 
thirty  times.  These  are  tested  on  the  morrow,  after  which 
two  series  (B2,  B'z)  are  read  and  tested  and  series  A3,  A's  are 
learnt.  Thus,  except  on  the  first  day,  four  series  are  read 
and  four  are  tested  daily.  Sometimes  the  test  by  the  saving 
precedes  that  by  the  scoring  method,  sometimes  the  contrary 


174 


EXPERIMENTAL  PSYCHOLOGY 


order  is  observed,  so  that  all  differences  due  to  temporal 
position  may  be  eliminated.  Each  day's  sitting  lasts  about 
fifty  minutes,  the  research  occupying  twenty-one  days.  The 
following  are  the  average  results  : — 


Series. 

Number  of  Repetitions 
to  relearn. 

r 

Tr 

A 

5-85 

0-9 

4503°" 

B 

9-6 

2-7 

1725°" 

In  this  experiment  it  will  be  observed  that  the 
associations  tested  in  the  A  series  are  twenty-four  hours 
old,  and  that  those  in  the  B  series  are  only  about  one  minute 
old.  The  results  show  that  in  the  younger  associations  B 
the  mean  association  strength  (as  judged  by  the  average 
scoring  times,  Tr,  and  by  the  average  number  of  scores  per 
series  r)  is  much  greater  than  in  the  older  associations  A. 
Nevertheless  the  B  series  require  many  more  repetitions 
than  the  A  series  in  order  that  they  may  be  relearnt.  If, 
now,  the  A  and  B  series  had  been  learnt  so  that  in  the  above 
experiments  they  gave  similar  values  for  r, — if,  in  other 
words,  the  number  of  original  readings  of  the  A  and  B  series 
had  been  respectively  increased  or  decreased,  so  that  the 
mean  association  strength  in  A,  after  twenty-four  hours,  was 
equal  to  that  in  B,  after  one  minute, — it  is  clear  that  the 
difference  between  them  in  the  number  of  repetitions  neces- 
sary for  relearning  would  have  been  still  greater.  Doubtless 
there  are  complex  factors  at  work  in  this  experiment  as  in 
the  last,  but  the  observed  differences  are  quantitatively  too 
great  to  lead  us  to  any  other  conclusion  than  that,  when 
two  associations  are  of  equal  strength,  but  of  unlike  age, 
repetitions  act  more  effectively  on  the  elder  than,  the 
younger. 

The  Influence  of  Time  on  Associations  of  Different  Age. — 


MEMORY  175 

Yet  another  factor  must  be  invoked  in  order  to  explain  the 
familiar  fact  that,  if  a  given  task  be  relearnt  on  successive 
days  by  the  saving  method,  until  it  can  on  each  day  be 
correctly  reproduced,  the  number  of  repetitions  necessary 
for  so  doing  grows  daily  less,  until  ultimately  no  fresh 
repetitions  are  needed  at  all.  We  are  compelled  to  assume, 
that  when  two  associations  are  of  equal  strength,  but  of 
unlike  age,  time  has  a  more  marked  effect  on  the  younger 
than  on  the  elder  association. 

[  The  Most  Economical  Method  of  Learning. — The  experi- 
ments on  the  distribution  of  repetitions  (page  171)  have  led 
to  a  more  detailed  investigation  of  the  most  economical  modes 
of  learning  a  given  task.  The  two  modes  of  learning  which 
have  most  frequently  been  compared  may  be  called  the 
"  entire  "  and  the  "  sectional." 1  In  the  entire  method,  the 
material  is  learnt  by  reading  it  completely  through  time 
after  time.  In  the  sectional  method,  the  material  is  sub- 
divided, and  the  subject  repeats  each  section  until  it  is 
learnt,  before  passing  on  to  learn  the  following  section 
(exp.  97). 

The  factors  determining  the  relative  efficacy  of  these 
two  methods  are  obviously  very  complex,  and  nearly  all  of 
them  must  vary  considerably  in  different  individuals.  On 
the  whole,  however,  the  experimental  evidence  is  in  favour  of 
entire  learning,  sectional  learning  proving  less  economical  the 
greater  the  number  of  sections  into  which  the  task  is  divided. 

It  will  be  noticed  that  in  the  entire  method  there  is 
a  total  absence  of  those  unnecessary  associations,  between 
the  end  and  the  beginning  of  the  same  section,  which  are 
inevitably  formed  in  sectional  learning.  When  the  material 
is  sensible,  the  entire  method  enables  a  general  impression 
of  the  whole  to  be  obtained,  while  every  subsequent  reading, 
by  improving  the  learner's  grasp  of  the  contents,  suggests 
fresh  rational  aids  to  memory.  In  the  sectional  method, 
attention  is  more  likely  to  wane  during  successive  repetitions 

1  The  " entire "  is  also  known  as  the  "global"  method. 


EXPERIMENTAL  PSYCHOLOGY 


than  in  the  entire  method.  On  the  other  hand,  the  subject 
gains  confidence  by  the  feeling  that  he  is  already  mastering 
at  least  a  part  of  the  task,  while  in  the  entire  method 
success  looms  vaguely  and  indistinctly  before  him.  A  mixed 
method  of  learning  has  been  investigated  in  which  the 
matter  is  learnt  in  three  sections,  and  after  each  section  has 
been  learnt  the  matter  is  recited  afresh  from  the  beginning. 
This  method,  although  more  economical  as  regards  immediate 
memory,  is  surpassed  by  the  entire  method  in  respect  of 
retention. 

When  the  task  consists  of  very  unfamiliar  matter, 
requiring  undue  strain  of  attention, — when,  for  instance, 
adults  learn  series  of  foreign  words,  or  when  children  learn 
series  of  senseless  syllables, — the  fatigue  involved  in  the 
entire  method  may  be  so  great  that  the  needful  repetitions 
are  more  numerous  than  in  the  sectional  method ;  yet  the 
series  is  better  retained.  For  like  reason,  a  subject  A,  who 
finds  considerable  difficulty  in  daily  learning  four  series  of 
syllables  in  trochaic  rhythm,  two  by  the  entire,  two  by  the 
sectional  method,  gives  the  following  relative  scores  and 
short  scoring  times,  as  compared  with  another  subject  By  to 
whom  the  task  comes  more  easily : — 


Subject. 

Entire. 

Sectional. 

r 

2V<2000ff 

r 

IY<2000°" 

A  (average  of  twenty 
days) 

0-30 

9 

0-44 

23 

B  (average  of  first 
eighteen  days) 

0-31 

21 

0-36 

21 

,,  (average  of  second 
eighteen  days) 

0-31 

16 

0-23 

11 

There  is  some  evidence  that  continued  practice  at  one 
or  other  of  these  methods  materially  alters  the  relation 


MEMORY  177 

between  them.  The  subject,  however,  is  too  complex  to 
be  pursued  further  here.  Individual  differences  in  the 
relation  of  subsidiary  to  principal  associations,  in  liability 
to  retro-active  inhibition  of  associations,  in  the  play  of 
perseverance  and  of  consolidation  changes,  besides  the 
factors  which  have  been  just  enumerated,  must  make  a 
precise  investigation  of  the  subject  extremely  difficult.] 

Improvement  in  Mechanical  Learning. — We  have  already 
pointed  out  (page  153)  how  necessary  it  is  to  begin  the 
experimental  study  of  memory  with  simple  meaningless 
material.  Yet  even  under  these  conditions  introspection 
shows  that  the  mental  processes  are  very  complex  and 
inconstant  until  practice  has  brought  about  a  uniform 
method  of  learning.  At  the  outset,  the  novice  is  disturbed 
by  diverse  extraneous  ideas  which  have  in  turn  to  be 
controlled.  He  makes  purposeless  movements  of  the  eyes, 
limbs,  and  face,  many  of  which,  being  the  outcome  of  an 
endeavour  to  restore  a  flagging  attention,  are  in  themselves 
distracting.  Indeed,  his  consciousness  is  continually  toned 
with  displeasure,  he  is  continually  changing  his  method 
of  learning,  now  laying  stress  on  the  absolute  position  of 
syllables,  now  marking  the  rhythm,  now  changing  the 
imagery  employed.  But  with  increasing  practice,  he  dis- 
covers the  best  method  of  learning.  His  attention  is  no 
longer  divided,  untoward  thoughts  and  unnecessary  move- 
ments cease,  his  available  fund  of  mental  energy  is  wholly 
devoted  to  the  task.  His  control  over  the  initial,  useless 
and  harmful,  factors  gives  him  a  pleasant  feeling  of  mastery, 
spurring  him  on  to  further  success.  Any  mnemonic  helps 
on  which  he  had  at  first  been  prone  to  rely  are  by  now 
discarded.  His  "general  attitude"  to  the  work  becomes 
increasingly  favourable.  He  yields  more  and  more  com- 
pletely to  sheer  mechanical  memory. 

Such  is  the  practice  which  must  needs  be  obtained 
before  experiments  with  meaningless  matter  can  give  truly 
reliable  results.  Yet  even  under  these  conditions  the 

12 


1 78  EXPERIMENTAL  PSYCHOLOGY 

attention  must  change  somewhat  with  different  readings. 
For  example,  during  the  first  reading  of  difficult  material 
it  is  almost  wholly  concentrated  on  the  pronunciation  of 
the  presented  matter.  Only  in  later  readings  is  it  given  up 
to  stamping  in,  to  impressing,  the  material. 

[  The  Influence  of  Speed  of  Heading. — The  type  of  imagery 
employed  is  liable  to  alter  with  change  in  the  rate  of  read- 
ing. There  is  some  evidence  that  auditory  imagery  and 
rhythmic  effects  usually  preponderate  in  rapid  reading. 
The  most  effective  speed  of  reading  is  naturally  dependent 
on  the  difficulties  of  the  material  and  on  the  kind  of  efficiency 
that  is  desired.  It  appears  that,  within  certain  limits,  slow 
readings  of  senseless  syllables  give  a  greater  number  of 
scores,  but  a  smaller  percentage  saving,  than  quicker  readings 
in  greater  number  during  the  same  period  of  time.  But  so 
far  we  are  without  sufficient  experimental  data  to  speak  at 
all  precisely  on  the  relations  between  the  rate  of  reading, 
the  rate  of  learning,  and  the  rate  of  forgetting.] 

The  Learning  of  Sensible  Matter. — The  learning  of  sensible 
matter  differs  from  that  of  senseless  matter,  in  that  it  is 
accompanied  by  a  great  number  of  associations  which  are 
wanting  in  simple  mechanical  learning, — save,  of  course, 
when  artificial  mnemonic  aids  are  employed.  While  sense- 
less syllables  are  linked  by  associations  only  in  time  and 
space,  the  learning  of  sensible  matter  involves  a  mastery 
not  only  of  matter  but  also  of  meaning,  the  effect  of  which 
is  to  knit  simpler  into  more  complex  units. 

Eational  learning  allows  of  the  formation  of  manifold 
associations,  which  when  acting  together  materially  facilitate 
the  desired  revival.  It  is  a  fully  established  principle  of 
psychology  that  while  a,  which  has  previously  been  associated 
with  6,  may  no  longer  be  able  to  reproduce  it,  yet  when  a  is 
given  along  with  x, — which  is  also  associated  with  &,  but  in 
too  weak  a  fashion  to  revive  it  unaided, — "b  will  be  repro- 
duced. This  summation  effect  is  of  undoubted  influence  in 
the  recollection  of  sensible  presentations. 


MEMORY  179 

We  have  already  pointed  out  (page  157)  that  the  limiting 
number  of  letters  and  of  senseless  syllables,  which  can  be 
immediately  reproduced  after  a  single  reading,  is  approxi- 
mately the  same  for  each.  But  just  as  a  syllable  is  viewed 
by  an  educated  person  as  a  unit,  although  composed  of 
several  different  letters,  so  for  the  purpose  of  memory  a 
group  of  words  or  a  short  phrase  becomes  a  unit  in  virtue 
of  its  meaning. 

We  have  also  examined  data  (page  158),  showing  how 
the  increase  in  length  of  a  series  of  senseless  syllables 
influences  the  number  of  repetitions  needed  for  immediate 
memory.  The  subject  of  those  experiments  states  that  he 
can  learn  six  stanzas  of  Byron's  Don  Juan  at  a  sitting  in 
fifty-two  repetitions.  Each  of  these  stanzas  contains  about 
eighty  syllables  in  about  thirty-six  words,  when  articles, 
prepositions,  pronouns  and  similar  dependent  words  are 
left  out  of  account.  But  he  has  experimentally  shown 
(page  158)  that  thirty-six  senseless  syllables  require  fifty- 
five  repetitions ;  whereas  a  single  stanza  of  poetry  requires 
about  eight  repetitions.  We  have  thus  some  measure  of 
the  astonishing  saving  effected  by  rational  associations  in 
the  learning  of  sensible  matter. 

The  superior  retentiveness  of  rationally  learnt  material 
is  familiar  to  every  one.  The  same  observer  found  that, 
whereas  he  lost  66*3  per  cent,  in  the  case  of  senseless 
syllables  (page  162),  with  stanzas  of  poetry  he  only  lost 
50  per  cent.,  in  relearning  twenty-four  hours  after  the 
original  learning.  With  longer  intervals  of  time,  this 
difference  in  retentiveness  between  rational  and  merely 
mechanical  learning  becomes  yet  more  striking.  It  has 
been  observed  chat,  even  twenty-two  years  after  a  piece 
of  poetry  has  been  learnt,  a  saving  of  7  per  cent,  may  be 
effected. 

The  Superiority  of  Rational  Learning. — Individuals  differ 
very  much  in  their  relative  use  of  mechanical  and  rational 
learning.  Those  who  have  unusually  strong  perseverance 


iSo  EXPERIMENTAL  PSYCHOLOGY 

tendencies  or  unusually  vivid  imagery,  are  able  to  depend 
very  largely  on  the  former  method,  especially  for  immediate 
memory.  Relying  on  mechanical  learning,  a  boy  will  master 
his  lesson  successfully  by  repeating  it  just  before  he  comes 
to  class,  and  an  actor  may  become  word  perfect  in  his  part 
at  a  few  hours'  notice.  But  folk  thus  gifted  with  good 
immediate  memory  are  apt  to  retain  what  they  have  learnt 
for  a  comparatively  short  time.  "  Digested  "  is  super%r  to 
"  crammed  "  learning  for  the  purposes  of  mediate  memory. 

The  Influence  of  Practice  and  Age  on  Memory.  —  We 
have  several  pieces  of  experimental  evidence  indicating  that 
memory  improves  with  practice,  and  that  at  least  immediate 
memory  improves  in  children  with  advancing  years.  We 
have,  however,  yet  to  determine  in  what  this  improvement 
of  memory  consists. 

If  we  accept  the  prevalent  view,  that  an  experience 
leaves  its  impress  on  the  brain,  just  as  a  seal  makes  its 
mark  on  wax,  it  is  open  for  us  to  suppose  that  the  depth  of 
such  impressions  (hence  their  retentiveness  or  repro- 
ducibility)  varies  with  the  practice  or  age  of  the  individual. 
Or,  believing  that  the  impressionability  of  brain  substance 
is  a  fixed  unalterable  inheritance  for  any  given  individual, 
we  may  suppose  that  whatever  later  improvements  in 
memory  occur  are  due  to  the  play  of  increasing  intelligence 
and  other  factors  influencing  the  general  attitude  of  the 
individual,  to  some  of  which  we  have  already  drawn 
attention. 

A  third  course,  however,  is  open  to  us.  We  may  with 
good  reason  dismiss  the  comparison  of  the  impressionability 
of  the  brain  with  that  of  wax  as  improbable ;  and  we  may 
prefer  to  express  the  phenomena  of  memory  solely  in  terms 
of  perseverance  tendencies  and  association  strengths.  We 
may  then  attribute  the  influence  of  practice  and  age  on 
memory  to  the  action  of  such  factors  as  we  have  just 
mentioned  upon  the  perseverance  tendencies  and  association 
strengths  of  experiences. 


MEMORY  181 

If  we  are  thus  inclined  to  regard  the  differences  in 
memory,  arising  from  practice  and  age,  as  due  not  to 
changes  in  the  congenital  impressionability  of  the  brain- 
substance,  but  principally  to  differences  of  mental  attitude 
inseparable  from  increasing  practice  and  age,  we  must  not 
overlook  the  more  direct  influence  of  physiological  conditions 
on  the  perseverance  tendency  and  the  association  strength 
of  experiences.  Drugs,  fatigue,  and  disease  may  un- 
questionably affect  both  these  factors.  The  falling-off  of 
memory  in  old  age  is  doubtless  chiefly  due  to  a  general 
decrease  in  association  strength  both  for  remote  and  for 
recent  experiences.  In  old  age,  associations  which  once 
were  easily  effective  become  only  effective  with  difficulty, 
while  those  which  were  with  difficulty  effective  now  pass 
altogether  below  the  threshold.  In  a  search  for  further 
determining  factors,  it  is  important  to  avoid  careless  con- 
fusion between  the  various  conditions  which  are  commonly 
involved  in  memory.  For  example,  the  psychological 
processes  which  effect  the  mere  "  recognition  "  of  an  experi- 
ence are  very  different  from  those  effecting  the  voluntary 
or  spontaneous  "reproduction"  of  an  experience. 

The  important  influence  of  what  we  have  called  the 
"  general  attitude  "  on  learning  is  indicated  by  experiments 
which  have  been  conducted,  with  the  object  of  determining 
whether  prolonged  practice  in  learning  senseless  syllables 
effects  improvement  in  subsequently  learning  sensible 
syllables,  numbers,  passages  of  prose  and  poetry,  and  other 
kinds  of  material.  It  appears  that  improvement  both  in 
speed  of  learning  and  in  retentiveness  occurs  for  all  kinds  of 
learning,  but  that  it  is  the  more  marked,  the  nearer  akin  be 
the  subsequent  material  to  that  on  which  practice  had  been 
bestowed. 

Individual  Differences  of  Memory.  —  Apart  from  the 
effects  of  age  and  practice,  individuals  show  differences 
dependent  on  the  interest  and  intelligence  which  they 
bring  to  their  work.  Moreover,  a  strong  perseverance 


1 82  EXPERIMENTAL  PSYCHOLOGY 

tendency  in  one  individual  will  rivet  his  attention  to  the 
task,  while  another,  in  whom  it  is  weak,  will  be  easily 
distracted  by  chance  impressions.  Again,  for  various 
reasons,  the  mean  association  strength  may  differ  in  any 
given  two  individuals. 

But  even  when  individuals  possess  the  same  mean  per- 
severance tendency  and  association  strength,  and  learn  the 
same  material  under  the  same  external  conditions,  neverthe- 
less their  tendency  or  ability  to  reproduce  that  material 
may  not  be  the  same,  owing  to  individual  differences  in 
the  play  of  other  conditions.  For  we  may  assume  that 
ideas  are  continually  struggling  among  one  another  to 
occupy  the  focus  of  consciousness,  and  that  there  is  a 
constant  rivalry  between  the  play  of  reproduction  effects 
and  the  ceaseless  inflow  of  sensory  impressions.  And  we 
may  suppose  that  the  suppression  of  adverse  perseverance 
tendencies  and  the  liability  of  associations  to  inhibition 
vary  with  different  individuals,  and  indeed  with  the  same 
individual  at  different  ages. 

BIBLIOGRAPHY. 

The  following  are  added  to  the  references  given  in  the  previous 
chapter:— A.  Jost,  "Die  Associationsfestigkeit  in  ihrer  Abhringigkeit  von 
d.  Vertheilung  d.  Wiederholungen,  Ztsch.  f.  Psychol.  u.  Physiol.  d.  Sinnes- 
organe,  1897,  xiv.  436.  L.  Steffens,  "  Experimentelle  Beitriige  zur  Lehre 
vom  okonomischen  Lernen,  ibid.  1900,  xxii.  321.  C.  Pentschew,  "Unter- 
suchungen  zur  Oekonomie  u.  Technik  d.  Lernens,"  Arch.  f.  d.  ges.  Psychol., 
1903,  i.  417.  E.  Ebert  u.  E.  Meumann,  "Ueber  einige  Grundfragen  d. 
Psychol.  d.  Uebungsphanomena  im  Bereiche  d.  Gedachtnisses,"  ibid.  1904, 
iv.  1.  W.  H.  Winch,  "Immediate  Memory  in  School  Children,"  Brit. 
Journ.  of  Psychol.,  1904,  i.  127;  1906,  ii.  52.  "The  Transfer  of  Improvement 
in  Memory  in  School  Children,"  ibid.  1908,  ii.  284.  P.  Ephrussi,  "Experi- 
mentelle Beitrage  zur  Lehre  vom  Gedachtniss,"  Ztsch.  f.  Psychol.  u.  Physiol. 
d.  Sinnesorgane,  1904,  xxxvii.  56,  161. 


CHAPTEK  XIV 
ON   MUSCULAR  AND   MENTAL  WORK  1 

The  Determination  of  Efficiency. — The  efficiency  of  an 
organ  is  determined  by  its  output  of  work  during  unit  periods 
of  time.  We  measure  the  efficiency  of  glandular  tissue  by 
observing  the  quality  and  quantity  of  its  secretion*,  and  by 
taking  into  account  the  conditions  under  which  the  secre- 
tion has  been  poured  forth.  We  measure  the  efficiency  of 
muflculartiesue  by  observing  the  extent  and  frequency  of 
its  contractions,  and  by  taking  into  account  the  conditions 
under  which  the  tissue  has  contracted,  e.g.,  the  resistance 
which  the  force  of  contraction  has  had  to  overcome.  Such 
tissues  or  organs  can  be  experimentally  isolated  from  the 
influence  of  other  tissues  or  organs ;  we  can  thus  simplify 
the  conditions  affecting  their  activity. 

(But  the  efficiency  of  the  nervous  system  is  a  more 
difficult  matter  of  investigation!  For,  so  far  as  nervous 
activity  manifests  itself  as  consciousness,  it  must  be  gauged 
by  the  nature  and  amount  of  mental  work  performed. 
And,  so  far  as  it  serves  unconsciously  to  co-ordinate  or  to 
direct  the  activities  of  other  tissues  of  the  body,  its  work 
cannot  be  adequately  studied  apart  from  its  effects  on  the 
tissues  which  it  controls. 

The  Interrelation  of  Mental  and  Muscular  Activity. — 
Desirable  as  it  is  to  base  our  knowledge  of  complex  activi- 
ties on  a  preliminary  study  of  simpler  ones,  we  must  not 
forget  that  the  .conditions  which  obtain  within  the  intact 

1  See  footnote  to  Chapter  III. 


1 84  EXPERIMENTAL  PSYCHOLOGY 

living  organism  are  very  different  from  those  which  are 
artificially  induced  by  the  experimental  isolation  of  organs 
to  which  we  have  just  referred.  We  shall  presently  see 
that  voluntary  muscular  work  is  determined  not  only  by 
local,  but  also  by  very  remote  and  by  general  conditions ; 
for  example,  that  it  is  closely  dependent  on  the_mental  con- 
dition of  the  individual  at  the  moment.  Similarly,  mental 
activity  cannot  be  isolated  from  muscular  activity ;  for 
every  idea  tends  to  gain  expression  in  movement,  and  every 
state  of  attention  or  of  emotion  is  largely  dependent  on 
movement.  Moreover,  the  presence  of  the  katabolic  pro- 
ducts of  muscular  activity  in  the  general  circulation  and 
the  central  strain  involved  in  any  volitional  muscular 
exertion,  materially  affect  the  efficiency  of  other  forms  of 
volitional,  e.g.  intellectual,  work. 

Obviously,  then,  muscular  and  mental  work  are  so 
closely  related  in  the  intact  organism  that  it  is  impossible 
properly  to  study  the  one  if  we  neglect  the  other.  We 
shall,  accordingly,  first  attempt  a  brief  analysis  of  the  con- 
ditions affecting  muscular  work. 


MUSCULAK  WOEK. 

Ergograpliy. — The  work  performed  by  an  active  muscle, 
whether  removed  from  or  intact  in  the  body,  may  be  best 
determined  by  means  of  graphic  records.  The  movements 
of  the  contractile  tissue  are  communicated  to  a  suitable 
lever,  which  accurately  registers  the  height  of  each  contrac- 
tion. This  lever  is  provided  with  a  writing  point,  which  is 
brought  to  bear  on  a  slowly  travelling  smoked  surface.  By 
such  means  an  "  ergogram,"  or  graphic  record  of  the  work 
done,  may  be  obtained.  The  height,  number,  and  frequency 
of  the  contractions  and  the  resistance  which  the  contrac- 
tions overcome  can  easily  be  deduced.  In  experiments  on 
voluntary  contraction  (exp.  98),  the  simplest  possible  move- 
ment is  usually  chosen,  e.g.  regular  flexion  and  extension  at 


MUSCULAR  AND  MENTAL  WORK    185 

a  single  finger  joint ;  the  weight  lifted  is  sufficiently  heavy 
to  tax  the  subject's  efforts  nearly  to  their  utmost;  the 
movements  of  the  finger  are  executed  at  a  prescribed  rhyth- 
mical rate,  and  they  are  graphically  registered  in  the  form 
of  an  ergogram  (fig.  4). 

Peripheral  and  Central  Factors  in  Muscular  Fatigue. — 
The  chemical  products  of  muscular  activity,  as  contraction 
follows  contraction,  accumulate  within  the  muscle  more 
rapidly  than  they  can  be  removed  by  the  blood  stream  or 
replaced  by  the  reconstruction  of  material  available  for 
fresh  work.  In  the  case  of  the  living  muscle,  isolated,  with 
its  motor  nerve,  from  the  organism,  a  state  of  total  muscular 
exhaustion  appears  to  be 
safeguarded  by  the  deli- 
cate constitution  of  the 
end  plate.  It  is  believed 
that  this  structure,  which 
forms  the  point  of  con- 
nection between  the  nerve 
and  the  muscle  fibre, 
suffers  fatigue  earliest. 
In  the  intact  living  body, 
however,  the  apparent  FlG  4 

fatigue  produced  by  voli- 
tional muscular  exercise  seems  to  have  a  more  central 
origin.  For,  when  a  series  of  contractions  have  been 
volitionally  obtained,  and  when  ultimately  no  further 
contractions  can  be  volitionally  produced,  they  may 
nevertheless  be  evoked  by  electrical  stimulation  of  the 
mpjtorjierve. 

It  has  been  suggested  that  this  apparently  central 
fatigue  of  the  intact  organism  has  its  seat  in  the  nerve 
cells  of  the  central  nervous  system,  which  saves  the  delicate 
end  plates  from  being  exhausted,  just  as  the  end  plates 
protect  the  muscle  fibre.  But  this  is  an  improbable  ex- 
planation. We  shall  presently  see  that  it  is  highly  improb- 


1 86  EXPERIMENTAL  PSYCHOLOGY 

able  that  the  nerve  cells  of  the  brain  or  cord  become 
exhausted  during  an  ergographic  record.  And  we  shall 
bring  forward  evidence  to  show  that  the  experience  of 
absolute  impotence  which  marks  the  close  of  an  ergographic 
tracing  is  due,  not  so  much  to  true  protoplasmic  fatigue,  as 
to  the  inhibitory  and  sensory  effects  of  impulses  which 
ascend  to  the  motor  centres  of  the  cortex  and  cord  along 
the  afferent  fibres  with  which  every  muscle  and  tendon  are 
supplied.  Let  us  first  consider  the  inhibitory  effects,  and 
their  importance  in  muscular  activity. 

The  Inhibitory  Effect  of  Afferent  Impulses. — It  is  well 
known  that  the  afferent  fibres,  running  from  the  muscular 
(and  other)  tissues  of  the  body  to  terminate  around  the 
cells  of  the  motor  nuclei  of  the  cord,  bulb,  and  mid-brain 
(page  74),  play  an  important  part  in  reflexly  co-ordinating 
muscular  action.  Experiments  have  repeatedly  proved  that, 
in  the  absence  of  these  afferent  impulses,  our  bodily  move- 
ments become  seriously  hampered  and  irregular.  The 
important  function  of  these  impulses  is  best  seen  in  the 
case  of  limb  movements.  Here  it  has  been  shown  that  the 
various  motor  centres  of  the  cord,  which  control  the  move- 
ments of  flexion  and  extension,  are  alternately  excited 
or  inhibited,  owing  to  the  play  upon  them  of  afferent 
impulses  peripherally  derived  from  the  movements  them- 
selves. The  contractions  of  one  (e.g.  a  flexor)  group  of 
muscles  reflexly  inhibit  the  simultaneous  activity  of  the 
antagonistic  (e.g.  extensor)  group  of  muscles,  the  extensor 
centres  of  the  cord  being  reflexly  inhibited  when  the 
antagonistic  flexor  centres  are  active,  and  vice  versa. 

The  Different  Effects  of  Constant  and  Variable  Loads. — 
The  effects  produced  on  muscular  work  by  these  afferent 
impulses  seem  to  be  largely  dependent  on  the  amount  of 
resistance  which  the  muscular  effort  has  to  overcome.  Let 
us  suppose  that  an  ergogram,  like  figure  4,  is  being  obtained 
by  regularly  flexing  and  extending  a  finger  to  its  maximal 
extent  every  two  seconds,  and  that  the  finger  has  to  raise 


MUSCULAR  AND  MENTAL  WORK          187 

a  load  of  five  kilograms.  After  a  short  time  the  finger  will 
be  in  a  state  of  apparently  complete  exhaustion ;  no  further 
voluntary  movement  can  be  obtained.  If  the  weight  of  five 
be  now  replaced  by  another  of  four  kilograms,  a  fresh 
excellent  ergogram  can  be  immediately  obtained.  Or  if  the 
weight  be  lightened  just  as  the  ergograph  curve  begins  to 
descend,  the  original  maximal  height  of  lifting  is  once  more 
attained;  and  by  successive  reductions  of  the  weight  a 
stage  may  at  length  be  reached  when  the  height  of  the 
rhythmical  contractions  can  be  maintained  at  a  constant 
level  for  some  hours  without  further  change  of  weight. 

The  decrements  by  which  the  weight  is  diminished  in 
this  method  of  using  a  variable  load  may  be  plotted  out  in 
the  form  of  a  curve.  Such  a  line  falls  rapidly  at  first,  and 
afterwards  more  slowly,  finally  reaching  a  fairly  constant 
level.  Obviously  it  is  of  very  different  form  from  the  out- 
line of  an  ergogram  obtained,  as  in  fig.  4,  by  using  a 
constant  load.  But  it  perhaps  gives  a  truer  picture  of  the 
course  of  muscular  fatigue.  Moreover,  it  corresponds  to 
the  curve  yielded  when  a  dynamometer  is  grasped  for  a 
prolonged  period  (exp.  155),  and  it  corresponds  to  the  curve 
of  work  calculated  from  each  of  a  series  of  ergograms,  when 
the  weight  lifted  is  constant,  a  sufficient  interval  of  rest 
being  allowed  between  consecutive  ergograms  to  permit  of 
recovery  from  the  corresponding  fatigue  effects. 

We  see,  then,  that  a  single  ergogram  obtained  by  the 
use  of  a  constant  load  records  the  onset  not  of  general 
fatigue,  but  of  fatigue  towards  a  special  set  of  circum- 
stances; the  condition  is  one  of  special  rather  than  of 
general  impotence,  and  is  attributable  in  part  to  the 
inhibitory  action  of  afferent  impulses  from  the  motor 
apparatus.  Probably  for  like  reasons,  a  new  series  of 
contractions  can  be  elicited  by  electrical  stimulation  of  a 
motor  nerve,  after  the  muscle  has  been  apparently  tired  out 
by  a  series  of  volitional  contraction  (page  185).  When,  at 
length,  such  a  muscle  no  longer  responds  to  electrical 


1 88  EXPERIMENTAL  PSYCHOLOGY 

stimulation,  it  may  once  again  be  thrown  into  contractions 
by  the  will. 

Accordingly,  we  are  led  to  suspect  that  muscular  fatigue 
in  the  intact  organism  bears  a  close  resemblance  to  muscular 
practice.  Just  as  a  practised  muscle,  when  put  to  strange 
uses,  may  fail  to  show  the  beneficial  effects  of  a  different 
previous  exercise,  so  an  apparently  fatigued  muscle  may  no 
longer  exhibit  signs  of  fatigue  when  it  begins  to  work  under 
different  conditions. 

The  Sensory  Effect  of  Afferent  Impulses. — Let  us  now 
briefly  consider  the  same  afferent  impulses  from  their 
sensory  aspect.  When  the  motor  apparatus  is  anaesthetic,  the 
usual  sensations  of  fatigue  are  absent,  and  muscular  activity 
can  be  abnormally  prolonged.  Those  sensations  of  fatigue 
are  perhaps  partly  due  to  the  accumulation  of  katabolic 
products  within  the  muscles,  for  we  know  that  they  are 
most  rapidly  dissipated  by  muscular  massage.  (.But,  at  all 
events,  we  must  always  carefully  distinguish  between  the 
subjective  symptoms  and  the  objective  signs  of  fatigue.]  To 
feel  fatigue  is  by  no  means  inconsistent  with  the  perform- 
ance of  increased  muscular  work  ;  the  former  is  never  a  safe 
criterion  of  the  latter. 

The  Influence  of  Affection  and  Interest.  —  Muscular 
efficiency  is  affected  by  any  sensory  impulses  which  change 
the  affective  state  of  the  organism  (page  334).  Increased 
interest  or  undue  mental  excitement  leads  to  the  per- 
formance of  an  abnormally  large  quantity  of  muscular  work. 
On  the  other  hand,  the  pain  which  is  sometimes  met  with, 
owing  to  the  confined  position  of  the  moving  part  in 
ergographic  work,  may  seriously  reduce  the  output  of 
muscular  work.. 

The    Influence    of   Mental    Fatigue. — On    physiological 
grounds   we   should   expect   that   the   presence  of   fatigue 
products,  whatever  their   origin,  would   be  detrimental  to 
voluntary  muscular  work.  (^  But  an  unusually  good  ergogram^ 
may,  in  many  instances   at  least,  be  obtained,  when  the 


MUSCULAR  AND  MENTAL  WORK          189 

contractions  are  preceded  by  a  prolonged  period  of  intel- 
lectual work  which  has  undoubtedly  involved  considerable 
fatigue.  J 

The  Complexity  of  the  Conditions  of  Muscular  Efficiency. — 
At  present  we  can  only  record,  we  cannot  satisfactorily 
account  for,  these  various  influences.  It  may  be  that  certain 
afferent  impulses,  which  under  ordinary  conditions  are 
able  to  limit  the  efficiency  of  a  previously  active  muscle, 
under  other  conditions  no  longer  discharge  this  protective 
function,  so  that  the  muscle  performs  a  preternatural 
amount  of  work.  It  may  be  that  undue  mental  excitement 
or  mental  fatigue  directly  induces  a  heightened  central 
motor  excitability. 

But  powerless  as  we  are  to  pronounce  definitely  on 
these  matters,  their  mere  mention  serves  to  show  how 
complex  are  the  factors  influencing  voluntary  muscular 
efficiency.  We  see  that  the  purely  muscular  system  is 
overruled  by  higher  and  still  higher  series  of  nervous 
functions.  We  see  how  practice  and  fatigue  in  muscular 
performance  within  the  intact  organism  are  dependent  both 
on  mental  and  on  extra-mental  factors.  We  obtain  a  hint, 
too,  of  the  nature  of  the  underlying  differences,  in  virtue  of 
which  one  and  the  same  muscular  task  is  performed  in  such 
various  ways  by  different  persons. 

MENTAL  WORK. 

Methods  of  Procedure. — There  are,  broadly  speaking,  two 
methods  of  measuring  mental  efficiency.  In  the  one,  a 
period  of  mental  work  is  interrupted  from  time  to  time  by 
a  certain  psychological  test,  and  the  mental  efficiency  of  the 
subject  is  estimated  by  the  degree  of  success  with  which  at 
different  times  he  performs  the  interpolated  test.  In  the 
other  method,  the  subject  is  confined  to  a  given  task 
throughout  the  period  of  investigation,  and  variations  in 
his  mental  efficiency  are  directly  deduced  from  the  varying 


EXPERIMENTAL  PSYCHOLOGY 

quality  or  quantity  of  his  work,  performed  in  like  intervals 
of  time  at  different  stages  of  the  investigation. 

The  Interpolation  Method. — The  tests  which  have  been 
used  for  interpolation  in  the  first  method  may  be  classed  as 
"  aesthesiometric,"  "  ergographic,"  and  "  mental." 

The  dSsthesiometric  Test. — The  sesthesiorneter,  merely  a 
form  of  Weber's  compasses,  serves  to  determine  the  spatial 
threshold  (page  231).  Now,  it  has  been  claimed  that  the 
smallest  distance  at  which  a  double  touch  can  be  dis- 
tinguished on  the  skin  serves  as  a  measure  of  the  mental 
fatigue  of  the  subject.  We  shall  later  (Chapter  xvii.)  draw 
attention  to  the  psychical  factors  influencing  the  spatial 
threshold;  and  we  may  at  once  concede  a  general  corre- 
spondence between  mental  fatigue  and  the  threshold  for  the 
discrimination  of  two  points.  But  it  would  be  ridiculous  to 
insist  that  the  threshold  affords  an  accurate  estimate  of  the 
absolute  degree  of  mental  fatigue  in  a  given  individual,  or 
that  it  allows  of  a  comparison  of  the  relative  degrees  of 
fatigue  in  different  individuals. 

The  Ergographic  Test. — The  use  of  the  ergograph  for 
estimating  mental  fatigue  is  open  even  to  still  more  serious 
objections.  We  have  seen  (page  188)  that  a  state  of  mental 
fatigue,  so  far  from  being  detected  by  muscular  inefficiency, 
is  compatible  with  the  production  of  an  unusually  good 
ergogram. 

The  Combination  Test. — The  following  "mental"  tests,  that 
have  been  employed,  involve  reading,  learning,  and  calcula- 
tion (exp.  100). 

In  the  "  combination  "  test  an  interesting  story  is  read 
by  the  subject,  in  which  certain  words  or  parts  of  words  are 
omitted.  The  subject  has  to  supply  these  omissions,  and 
his  efficiency  is  estimated  by  the  amount  that  he  reads,  by 
the  correctness  of  the  words  he  supplies,  and  by  the  number 
of  words  which  he  has  omitted  to  supply. 

The  Letter-erasing  Test. — In  the  "letter-erasing"  test 
the  subject  is  told  to  read  through  the  pages  presented  to 


MUSCULAR  AND  MENTAL  WORK          191 

him,  and  to  cross  out  every  example  of  a  specified  letter. 
His  efficiency  is  estimated  by  the  amount  of  matter  read, 
and  by  the  number  of  occasions  on  which  the  specified 
letter  has  escaped  his  notice. 

The  Learning  Test. — In  the  "  learning  "  test  some  simple 
material  (a  series  of  letters,  figures,  or  syllables)  is  presented, 
which  the  subject  has  to  commit  to  memory.  His  mental 
efficiency  is  determined  by  the  number  of  repetitions  which 
are  required  before  the  series  has  been  learnt. 

The  Calculation  Test. — In  the  "  calculation "  test  the 
subject  has  to  effect  a  series  of  simple  additions  or  multiplica- 
tions, the  number  of  figures  added  or  multiplied  serving  as 
an  index  of  his  efficiency. 

The  Relative  Reliability  of  the  Methods.  —  This  first 
method,  which  we  have  been  describing,  the  method  of 
interpolating  tests,  is  for  many  reasons  less  satisfactory  than 
the  second  (or  "  continuous ")  method.  We  cannot  legiti- 
mately assume  that  the  test  measures  the  impaired  efficiency 
brought  about  by  quite  another  piece  of  mental  work.  The 
interpolation  of  a  test  always  involves  a  change  of  interest, 
and  thus  a  disturbing  factor,  favourable  or  unfavourable  to 
the  test,  at  once  appears  on  the  scene.  Moreover,  the 
attitude  of  the  subject  towards  the  test  must  be  widely 
different  at  different  applications,  owing  to  increasing 
practice  and  other  causes.  In  consequence,  this  method 
is  not  so  well  fitted  for  an  accurate  study  of  the  conditions 
affecting  mental  efficiency  as  the  second  method,  in  which 
the  subject  labours  uninterruptedly  at  a  given  task,  and 
his  power  of  work  is  estimated  by  the  amount  and  accuracy 
of  his  output  in  equal  intervals  of  time. 

The  Continuous  Method. — For  the  second  method  any  of 
the  mental  tests  just  described  are  available.  Most  of  the 
results,  however,  to  which  we  shall  allude  have  been  obtained 
by  use  of  the  calculation  method ;  the  subject  having  to  add 
successive  pairs  of  printed  single  figures  together,  and  to 
draw  a  line  at  the  particular  figure  reached  by  him  upon 


192  EXPERIMENTAL  PSYCHOLOGY 

hearing  a  signal,  which  is  sounded  every  minute,  or  every 
five  minutes. 

The  Mental  Work  Curve. — By  this  method  we  are  able 
to  construct  a  work  curve  (fig.  5) ;  the  divisions  on  the 
abscissa  denoting  successive  equal  intervals  of  time,  the 
ordinates  giving  the  amount  of  work,  i.e.  the  number  of 
additions  performed  during  successive  intervals.  The 
number  of  errors  in  such  simple  sums  is  found  to  be 
negligibly  small. 

Fatigue  and  Practice. — The  curve  thus  obtained  at  a 
single  sitting  may  or  may  not  rise  at  first,  but  sooner  or 
later  it  cannot  fail  to  fall  owing  to  increasing  fatigue.  It  is 
true  that  along  with  increasing  fatigue  goes  increasing 

practice ;  so  far  as  we 
^  know,  the  two  are  in- 

separable. But  it  is 
chiefly  in  the  early 
stages  of  work,  when 
the  gain  by  practice 

1     2     3     4     5     6     7     8     9    10    11    12        GXCCeds      the      10SS      by 

FIGt  5  fatigue,  that  the  work 

curve  rises. 

The  influence  of  practice  is  most  evident  at  the  early 
stages  of  experience.  Thus  during  twenty-six  days'  exercise 
at  simple  addition  an  average  daily  gain  of  12'2  per  cent., 
due  to  practice,  was  obtained  during  the  first  ten  days, 
whereas  for  the  next  ten  days  it  amounted  only  to  2*6  per 
cent.,  and  for  the  last  six  days  it  fell  to  1*9  per  cent.  This 
decrease  in  the  increment  contributed  by  practice  is  to  be 
observed  when  a  series  of  consecutive  daily  work  curves  are 
compared.  Owing  to  the  diminishing  practice  gain  in  the 
latter  curves,  the  fatigue  effect  shows  itself  sooner  than  in 
the  earlier  curves. 

The  Factors  in  Fatigue. — Whereas  the  practice  gain 
during  an  uninterrupted  task  grows  less  and  less,  the 
fatigue  increases  indefinitely  until  a  limit  may  be  reached 


370- 


360 


MUSCULAR  AND  MENTAL  WORK          193 

when  no  further  work  can  be  performed.  A  small  part  of 
the  fatigue  involved  in  the  addition  of  figures  is  of  muscular 
origin ;  the  hand  tires  of  its  cramped  position  and  of  frequent 
writing.  In  some  experiments  this  source  of  fatigue  has 
been  eliminated  by  excusing  the  worker  from  writing  down 
his  arithmetical  results  ;  but  such  a  procedure  restricts  the 
experimenter  to  unusually  reliable  subjects.  Even  then 
the  effects  of  eye  strain  remain,  which,  in  some  individuals 
at  least,  contribute  not  a  little  to  their  fatigue.  When, 
instead  of  the  addition  test,  typewriting  or  some  other  form 
of  manual  activity  (e.g.  marking  the  dots  already  figured  on  a 
rapidly  unwinding  roll  of  paper)  is  employed,  muscular 
fatigue  becomes  still  more  prominent.  With  adequate 
practice,  however,  it  nearly  or  altogether  disappears.  More- 
over, by  increasing  the  hardness  of  the  mental  task,  e.g.  by 
multiplying  series  of  three,  four,  or  more  consecutive  single 
figures  "  in  the  head,"  it  is  possible  to  reduce  the  degree  of 
muscular  fatigue  and  at  the  same  time  to  enhance  the  mental 
fatigue  involved. 

[From  the  physiological  standpoint  there  are  conceivably 
two  chief  sources  of  mental  (as  of  muscular)  fatigue,  the  one 
arising  locally  from  the  actual  wear  and  tear  of  the  part 
exercised,  the  other  more  widely  spread,  and  resulting  from 
the  presence  of  harmful  waste  products  in  the  general 
circulation.  The  former  possible  source  of  fatigue  is  re- 
movable by  increased  tissue  repair,  the  latter  by  a  better 
regulated  excretion  of  waste  products.  At  present  we  are 
ignorant  of  any  differences  that  may  exist  in  the  psycho- 
logical expression  of  these  two  physiological  factors. 

Indeed,  it  is  quite  likely  that  one  of  them,  namely, 
excessive  wear  of  the  nervous  system,  is  comparatively 
unimportant  within  the  limits  of  normal  health,  and  that 
its  influence  is  usually  safeguarded  by  other  factors.  At  all 
events,  we  must  bear  in  mind  the  possibility  that  many  of 
the  signs  of  mental  fatigue  may  be  simulated,  either  through 
the  play  of  inhibitory  nervous  impulses, — as  in  the  case  of 
'3 


194  EXPERIMENTAL  PSYCHOLOGY 

muscular  fatigue  (page  186), — or  owing  to  the  finally  success- 
ful competition  of  rival  processes  which  have  previously  been 
held  in  check  to  the  advantage  of  the  processes  exercised. 
It  may  be  urged  that  such  factors  are  referable,  on  the 
psychological  side,  to  experiences  of  boredom  rather  than  of 
true  fatigue.  But  boredom,  the  weariness  produced  by 
monotony  of  pursuit,  is  so  nearly  related  to  fatigue,  the 
outcome  of  exhausting  mental  work,  that  in  the  present 
state  of  experimental  psychology  their  separation  is  im- 
possible.] 

Spurts. — Were  the  amount  of  work,  as  shown  in  the 
mental  work  curve,  solely  an  expression  of  the  two  opposing 
factors  of  practice  and  fatigue,  we  should  expect  a  far 
smoother  curve  than  that  which  we  customarily  obtain. 
We  have  to  explain  the  irregularities,  the  various  peaks  and 
depressions  which  it  contains.  These  are  no  doubt  the 
results  of  distraction,  flagging  interest  and  increasing 
fatigue,  which  either  unconsciously  or  when  brought  to  the 
notice  of  the  worker  lead  to  momentary  "spurts,"  i.e.  to 
increase  in  the  volitional  strain  or  tension  which  he  brings 
to  bear  on  his  task. 

At  two  periods  in  the  course  of  the  work  curve,  spurts 
of  a  slightly  different  causation  are  frequently  met  with. 
The  one  occurs  at  the  very  start  of  a  task,  when  the  subject 
is  fresh  and  brings  to  his  work  a  marked  degree  of  volitional 
tension  which  it  is  impossible  for  him  to  maintain  for  many 
minutes.  The  other  occurs  when  he  feels  he  is  nearing  the 
end  of  his  task.  The  "initial  spurt"  is  experimentally 
unavoidable,  but  the  "end  spurt"  may  be  prevented  by 
irregularly  varying  the  duration  of  the  subject's  work  from 
day  to  day. 

A  little  consideration  will  convince  us  that  it  is  this 
ever  variable  factor  of  volitional  tension  which  is  chiefly 
affected  by  fatigue.  We  can  hardly  suppose  that  the  time 
involved  in  the  sheer  mechanical  addition  or  multiplication 
of  two  figures  materially  lengthens  as  fatigue  increases. 


MUSCULAR  AND  MENTAL  WORK          195 

It  is  rather  the  difficulty  of  concentrating  our  attention 
on  the  figures,  and  of  realising  their  meaning,  that  brings 
about  the  diminishing  output  of  our  work. 

Incitement. — In  addition  to  practice,  fatigue,  and  spurt, 
a  fourth  factor  is  recognisable  in  any  work  curve.  To  this 
we  may  give  the  name  "  incitement."  It  occurs  at  the  start 
of  work  after  a  period  of  previous  rest.  We  may  compare 
our  working  selves  to  machines  which  start  with  a  certain 
amount  of  inertia,  and  require  "  warming  up "  before  they 
reveal  their  true  efficiency.  The  result  of  the  loss  of 
incitement  is  familiar  enough,  when  we  return  to  a  task 
from  which  we  have  been  called  away,  even  for  a  few 
minutes.  What  is  missing  is  something  quite  different 
from  a  loss  of  practice.  The  human  machine  has  "  grown 
cold  "  during  the  interval.  The  loss  in  the  output  of  work 
after  such  a  period  of  rest  is  due  to  the  absence  of  this 
factor  of  incitement. 

[Adaptation. — Between  the  effects  of  incitement  and 
"adaptation"  it  is  difficult  to  draw  any  fast  line.  Yet 
Krapelin,  to  whom  we  are  chiefly  indebted  for  this  analysis 
of  the  work  curve,  has  made  endeavours  to  distinguish  them. 
While  the  results  of  the  loss  of  incitement  show  themselves 
after  even  a  short  rest  or  at  the  beginning  of  each  day's 
task  daring  an  investigation  consisting  of  several  consecutive 
days'  work,  the  results  of  lack  of  adaptation  are  to  be 
recognised  at  the  very  outset,  i.e.  on  the  first  day  of  such 
a  protracted  investigation  or  upon  the  recommencement  of 
work  after  several  days  of  rest.  The  process  of  adaptation 
consists  chiefly  in  acquiring  neglect  of  distracting  impressions 
which  so  seriously  divert  the  attention  of  the  inexperienced.] 

The  Effect  of  Rest  on  Work. — The  effect  of  rest  upon 
mental  efficiency  depends  inter  alia  upon  the  length  of  the 
rest.  It  may  be  readily  studied  by  comparing  the  output 
of  work  before  and  after  a  rest  pause.  A  very  brief  pause 
is  unfavourable,  owing  to  loss  of  incitement.  A  somewhat 
longer  pause  becomes  favourable,  owing  to  the  loss  of 


1 96  EXPERIMENTAL  PSYCHOLOGY 

fatigue  which  had  been  produced  by  the  preceding  period 
of  work.  But  although  the  work  done  after  a  pause  may 
be  benefited  by  the  loss  of  fatigue,  it  may  deteriorate, 
on  the  other  hand,  owing  to  the  loss  of  practice  during  the 
pause. 

Obviously,  then,  the  relation  between  the  work  done 
before  and  after  a  pause  varies  with  the  length  of  the  pause. 
A  pause  may  be  experimentally  found  of  such  length  that 
the  work  done  after  is  equal  to  the  work  done  before  the 
pause.  We  may  term  this  the  "  equilibrial  pause."  Again, 
another  length  of  pause  may  be  determined,  the  work  done 
after  which  reaches  a  higher  value,  in  relation  to  the  work 
done  before  it,  than  the  work  done  after  any  other  length 
of  pause.  We  may  term  this  the  "  most  favourable  pause." 

[The  Determination  of  Fatig ability. — The  increased  output 
resulting  from  the  most  favourable  pause  has  been  employed 
in  the  following  way  as  a  measure  of  fatigability.  The 
percentage  of  improvement  obtained  by  the  most  favourable 
pause  is  applied  to  a  like  period  of  work  in  which,  however, 
no  pause  whatever  has  occurred ;  and  the  fatigability  of  the 
subject  is  estimated  by  comparing  the  calculated  with  the 
observed  output  of  work  during  the  second  half  of  the  pause- 
less  period.  For  example,  let  us  suppose  that  the  most 
favourable  pause  after  a  half-hour's  addition  has  resulted 
in  a  subsequent  half-hour's  addition  of  2449  figures,  that  is, 
in  an  improvement  of  3*1  per  cent.,  compared  with  the 
addition  of  2376  figures  before  the  pause.  This  improve- 
ment, based  as  it  should  be  not  on  a  single  determination, 
but  on  the  mean  of  a  number  of  determinations,  is  then 
applied  to  the  work  done  on  the  first  half-hour  of  pauseless 
days,  i.e.  of  days  on  which  the  worker  added  uninterruptedly 
for  an  hour.  Let  us  suppose  that  the  mean  number  of 
additions  for  the  first  half -hour  of  the  pauseless  days  is 
2380.  Then  we  should  expect  an  improvement  of  3'1  per 
cent,  had  the  most  favourable  pause  been  interpolated; 
that  is,  we  should  expect  the  second  half-hour  to  yield  an 


MUSCULAR  AND  MENTAL  WORK          197 

output  of  nearly  2454  additions.  As  a  matter  of  fact,  the 
mean  value  of  the  output  during  the  second  half-hour  of 
the  pauseless  days  turns  out  to  be  2391.  The  difference 
between  the  calculated  and  the  observed  values,  and  the 
ratio  of  that  difference  to  the  absolute  amount  of  work 
done,  have  been  used  as  a  measure  of  the  fatigability  of  the 
subject.] 

[The  Determination  of  Improvability. — The  improvability 
of  individuals  has  been  measured  by  comparing  the  efficiency 
of  the  work  done  during,  say,  the  first  half-hour  of  con- 
secutive days.  There  appears  to  be  a  definite  relation 
between  the  amount  of  gain  from  practice  and  the  degree 
of  the  subject's  fatigue.  As  with  increasing  practice  the 
daily  practice  gain  diminishes,  so  the  fatigue  effect  diminishes 
in  the  daily  work  curve.] 

[The  Determination  of  Eetentiveness  of  Improvement. — The 
loss  of  practice  effects,  produced  by  disuse,  is  familiar  to  us 
all.  At  first  the  loss  is  extremely  rapid,  later  it  becomes  less 
and  less.  Thus,  while  a  pause  of  one  day  causes  a  consider- 
able drop  in  the  number  of  additions  performed,  the  effect 
of  a  pause  of  six  days  has  been  said  hardly  to  differ  from 
that  of  a  pause  of  four  months.  Individuals,  however,  vary 
in  their  ability  to  retain  practice  effects  already  acquired. 

Attempts  have  been  made  to  measure  the  retention  of 
practice  by  use  of  the  data  afforded  by  the  most  favourable 
pause.  It  is  assumed  that  practically  all  fatigue  has  dis- 
appeared during  the  most  favourable  pause,  and  that  the 
percentage  improvement  yielded  by  it  gives  a  measure  of 
the  gain  in  practice  derived  from  the  period  of  work  which 
immediately  preceded  the  pause.  That  percentage  is  now 
applied  to  the  work  done  after  the  most  favourable  pause, 
and  the  calculated  value  is  compared  with  the  actual  value 
next  obtained  from  the  worker  after  a  relatively  long  period 
of  rest.  The  difference  between  the  two  values  has  been 
used  as  a  measure  of  the  loss  of  practice  gain  during  that 
period  of  rest, 


198  EXPERIMENTAL  PSYCHOLOGY 

For  example,  a  subject  who  improves  by  4*8  per  cent- 
after  the  most  favourable  pause,  does  2916  additions  after 
that  pause.  Then,  if  that  improvement  represented  the 
practice  gain,  and  if  that  practice  effect  were  maintained, 
it  is  argued  that  the  subject  should  add  3055  figures  (i.e. 
4'8  per  cent,  more  figures  than  2916)  on  the  next  occasion. 
Let  us  suppose  that,  twenty-four  hours  later,  the  subject 
works  for  a  half -hour,  but  only  succeeds  in  adding  2868 
figures.  Then  the  difference,  namely,  187  figures,  represents 
his  daily  practice  loss.  A  comparison  of  the  results  of 
different  individuals,  it  is  urged,  yields  a  measure  of  indi- 
vidual differences  in  the  retention  of  practice.] 

[The  Meaning  of  the  Most  Favourable  Pause.  —  In  the 
face  of  the  important  assumptions  that  are  involved,  and 
the  many  complicated  factors  which  are  at  work,  it  is 
scarcely  necessary  to  point  out  that  such  calculations  can 
only  be  accepted  with  extreme  caution.  For  the  length 
of  the  most  favourable  pause  has  at  most  an  approximate 
value,  varying  with  the  condition  of  the  individual  and 
with  the  length  and  nature  of  the  task. 

Even  in  the  most  favourable  pause  we  cannot  assume 
that  all  fatigue  has  been  lost.  It  merely  expresses  the  most 
advantageous  balance  obtainable  between  factors  favourable 
and  detrimental  to  efficiency.  Further,  the  percentage 
gained  after  the  most  favourable  pause  depends  not  merely 
on  the  amount  of  the  worker's  previous  fatigue,  but  also  on 
the  speed  with  which  it  passes  away.  Owing  to  individual 
differences  in  the  rate  of  recovery  from  fatigue,  two  subjects, 
in  whom  equal  degrees  of  practice  and  fatigue  had  been 
produced,  may  yield  very  different  percentages  of  improve- 
ment after  the  most  favourable  pause.  Lastly,  the  neglect 
of  the  help  and  corroboration  afforded  by  introspection,  and 
the  procedure  of  applying  calculated  to  expected  results,  are 
obviously  fraught  with  danger  when  we  are  concerned  with 
psychical,  and  not  merely  with  physical  data.] 

Criticism  of  Laboratory   Work. — It  may  be  urged   that 


MUSCULAR  AND  MENTAL  WORK          199 

such  trivial  work  as  adding  pairs  of  figures,  marking  dots  or 
crossing  out  specified  letters,  bears  so  little  resemblance  to 
the  ordinary  operations  of  the  mind,  that  we  cannot  expect 
to  apply  the  results  thus  gained  by  laboratory  experiment 
to  the  conditions  of  daily  life.  Unquestionably,  our  work 
outside  the  laboratory  has  not,  as  a  rule,  that  monotonous 
simplicity  which  experiment  demands  for  the  purposes  of 
accurate  measurement.  The  mind  is  not  merely  a  machine, 
and  its  efficiency  cannot  be  calculated  as  if  it  were  an 
engine,  making  so  many  revolutions  per  minute.  We  have 
to  take  into  account  the  important  work  involved  in  the 
guiding,  directive  activity  of  the  mind.  In  ordinary  life  a 
great  part  of  our  mental  activity  is  occupied  not  so  much  in 
directly  producing  work  as  in  elaborating  and  satisfying  a 
procession  of  ends.  We  are  always  forming  and  realising  ends, 
which  in  turn  are  not  final  but  are  a  means  to  greater  ends, 
and  so  on.  Mental  activity  is  therefore  analogous  not 
merely  to  the  wear  and  tear  of  a  machine,  but  also  to  the 
directive  agency  of  the  engineer. 

Compared  with  its  importance  in  everyday  life,  this 
factor  of  directive  agency  plays  an  insignificant  part  in  the 
laboratory  task  of  marking  dots  or  of  adding  pairs  of  figures. 
We  have  only  to  practise  an  hour's  such  addition  work,  say 
for  ten  consecutive  days,  to  discover  how  automatically  the 
work  comes  to  be  carried  out,  despite  every  effort  to  prevent 
automaticity.  The  same  holds  to  a  surprising  degree  even 
with  much  harder  tasks,  e.g.  multiplying  series  of  three 
figures  "  in  the  head."  Our  thoughts  insist  on  wandering 
into  other  channels,  while  the  dull  task  of  adding  or 
multiplying  proceeds  quite  happily  without  appreciable 
help  from  consciousness.  From  time  to  time  we  recall 
ourselves  to  the  task, — whether  always  to  the  improvement 
of  the  latter  being  uncertain. 

To  this  objection  we  can  only  reply  that  experiment 
must  necessarily  start  from  the  simplest  conditions,  and 
that  just  as  the  other  abstract  sciences,  when  at  length  they 


200  EXPERIMENTAL  PSYCHOLOGY 

are  employed  in  applied  science,  approach  more  and  more 
closely  to  the  concrete  of  ordinary  experience,  so  psychology, 
once  having  gained  the  necessary  knowledge  under  simpler 
conditions  of  experiment,  may  some  day  be  enabled  to 
devise  experiments  more  complex  and  more  analogous  to 
the  conditions  of  workaday  life. 


BIBLIOGRAPHY. 

A.  Oehrn,  ' '  Experimentelle  Studien  zur  Individual  psychologic," 
Psychol.  Arbeiten,  1896,  i.  92.  E.  Araberg,  "  Ueber  d.  Einfluss  d.  Arbeits- 
pausen  auf  d.  geistige  Leistungsfahigkeit,"  ibid.,  300.  W.  H.  R.  Rivers 
and  E.  Krapelin,  "Ueber  Ermiidimg  u.  Erholung,"  ibid.,  627.  H. 
Ebbinghaus,  "  Ueber  eine  neue  Methode  zur  Priifung  geistiger  Fahigkeiten, 
u.  ihre  Anwendung  bei  Schulkindern,"  Ztsch.  f.  Psychol.  u.  Physiol.  d. 
Sinnesorgane,  1897,  xiii.  401.  A.  Binet  et  V.  Henri,  La  Fatigue  Intellectuelle, 
Paris,  1898.  E.  H.  Lindley,  "Ueber  Arbeit  u.  Ruhe,"  Psychol.  Arbeiten, 
1900,  iii.  482.  J.  Joteyko,  "Participation  des  centres  nerveux  dans  les 
phenomenes  de  fatigue  musculaire, "  L'Annee psychol.,  7meannee,  1901,  161. 
Z.  Treves,  "Ueber  die  Gesetze  d.  willkiirlichen  Muskelarbeit,  Arch.f.  d.  ges. 
Physiol. ,  1910,  Iviii.  ;  "  Le  Travail,  la  Fatigue  et  1'Effort,"  L'Annee  psychol., 
1906,  12me  annee,  34.  E.  Krapelin,  "  Die  Arbeitscurve,"  Philosoph.  Stud., 
1902,  xix.  459.  A.  Mosso,  Fatigue,  Eng.  trans.,  London,  1904.  W.  H.  R. 
Rivers,  The  Influence  of  Alcohol  and  other  Drugs  on  Fatigue,  London,  1908. 


CHAPTEK   XV 
ON  THE   PSYCHO-PHYSICAL  METHODS1 

The  Uses  of  the  Methods. — The  principal  object  of  these 
methods  is  to  determine  the  /conditions  of  our  experiences 
of  equality  and  differenced  This  forms  a  most  important 
theme  of  research  in  experimental  psychology.  [For  experi- 
ences of  equality  and  difference  underlie  (i.)  the  determina- 
,  tion  of  the  "  differentiaj^threshold,"  in  which  the  smallest 
"appreciable  difference,  produced  by  two  different  stimuli,  is 
ascertained. 

The  same  experiences  are  in  great  measure  involved  in 
(ii.)  the  determination  of  the  "  absolute  threshold "  of  an 
experience  (the  presentation,  for  example,  being  one  of 
minimal  intensity  or  quality,  or  one  of  minimal  temporal  or 
spatial  extent)  ;  for  here,  as  we  shall  subsequently  see,  the 
subject's  attitude  is  essentially  one  of  discrimination  between 
the  just  appreciable  and  the  inappreciable. 

Experiences  of  equality  and  difference  are  also  involved 
in  determining  the  conditions  of  apparent  equality  between 
two  experiences.  Apparent  equality  may  relate  simply  to 
(iii.)  the  experiences  of  two  objects ;  as  for  example  in  the 
determination  of  equality  in  the  lengths  of  two  lines  or  in 
the  brightnesses  of  two  lights. 

On  the  other  hand,  it  may  relate  to  (iv.)  the  experience 
of  the  differences  between  two  pairs  of  objects ;  as  for 
instance  when  we  determine  that  the  difference  between 
the  loudneeses  a  and  I  of  one  pair  of  sounds  is  equal  to  the 

1  See  footnote  to  Chapter  III, 


202  EXPERIMENTAL  PSYCHOLOGY 

difference  between  the  loudnesses  &  and  c  (or  c  and  d)  of 
another  pair  of  sounds. 

In  none  of  these  four  lines  of  research  can  we  strictly  be 
said  to  measure  subjective  experiences  of  equality  or  differ- 
ence. I  We  can  only  measure  the  objective  intensities  (or 
extents)  of  stimuli  which  produce  those  experiences. )  Hence, 
speaking  in  terms  of  physical  stimuli,  in  place  of  psychic 
experience,  we  may  say  that  these  four  lines  of  research 
respectively  involve  the  determination  of  (i.)  the  differential 
threshold  of  the  stimulus,  (ii.)  the  absolute  threshold  of  the 
stimulus,  (iii.)  the  apparent  equality  of  stimuli,  and  (iv.)  the 
apparent  equality  of  stimulus  differences. 

For  the  investigation  of  these  four  problems  three 
so-called  "psycho-physical"  methods  have  been  devised, 
intimate  familiarity  with  which  is  essential  for  reliable 
experiment.  They  are  "  the  method  of  mean  error,"  "  the 
limiting  method,"  and  "  the  constant  method.1*] 

THE  METHOD  OF  MEAN  ERROR. 

This  method,  sometimes  called  "  the  method  of 
production,"  is  especially  useful  in  determining  the  con- 
ditions of  equality  between  two  experiences.  Taking  a 
simple  case,  let  us  suppose  that  we  have  to  deal  with 
two  visual  stimuli,  each  of  which  consists  of  a  horizontal 
straight  line,  one  having  to  be  made  equal  to  the  other 
(exp.  101).  These  lines  might  conceivably  be  exposed  to 
the  subject,  either  simultaneously  or  successively,  either 
instantaneously  or  for  a  relatively  long  period.  Let  us 
suppose  that  they  are  exposed  simultaneously  for  a  prolonged 
period,  and  that  one  of  them,  which  we  shall  call  the 
"  standard  "  line,  lies  at  the  same  horizontal  level  as  the 
other  line  which  is  the  "  variable."  The  subject  is  directed 
(by  aid  of  some  simple  contrivance)  to  change  the  length  of 
the  variable  line  until  it  appears  to  him  equal  to  the 
standard  line. 


THE  PSYCHO-PHYSICAL  METHODS        203 

The  Crude  Constant  and  Crude  Average  Errors. — If  a 
series  of  such  determinations  of  equality  be  made  by  the 
subject  and  recorded  by  the  experimenter,  the  difference 
between  the  mean  of  these  determinations  and  the  length  of 
the  standard  gives  the  mean  error  of  estimation.  This  is 
known  as  the  "  crude  constant  error,"  ec.  Its  reliability 
depends  on  the  deviation  of  the  individual  determinations 
from  one  another;  this  can  be  measured  by  the  mean 
variation  or  mean  variable  error  (page  124).  The  "crude 
average  error"  is  calculated  by  finding  the  mean  of  the 
deviations  of  the  individual  determinations  from  the 
standard,  regardless  of  algebraic  sign. 

These  data,  however,  would  give  us  no  knowledge  of  the 
various  factors  which  contribute  to  the  error  of  estimation. 
We  must  not  be  content  with  the  blurred  result  arising 
from  such  an  unpsychological  mode  of  procedure.  We 
must  endeavour  to  analyse  the  factors  and  to  refine  our 
methods  of  experiment,  so  that  under  certain  conditions  the 
influence  of  one  factor,  under  other  conditions  the  influence 
of  another  factor,  may  be  brought  to  light  and  specially 
investigated. 

Thus,  in  the  above  simple  experiment,  the  subject  was 
presented  with  a  variable  line  which  he  had  to  make  equal 
to  the  standard  line.  But,  we  may  well  ask,  is  the  result 
independent  of  whether  the  variable  line,  when  presented  to 
the  subject,  were  shorter  or  longer  than  the  standard  line 
with  which  it  had  to  be  equated  ?  This  question  can  only 
be  answered  by  making  one  set  of  observations  in  which  the 
subject  has  to  lengthen  the  variable  line,  and  another  set  in 
which  he  has  to  shorten  it. 

The  Space  and  Time  Errors. — Moreover,  in  the  above 
experiment  we  have  not  specified  on  which  side  the  variable 
lies  with  regard  to  the  standard  line.  Consequently,  we 
have  also  to  determine  the  constant  error  for  a  series  of 
observations  in  which  the  former  line  lies  to  the  right  and 
for  another  series  in  which  it  lies  to  the  left  of  the  latter, 


204  EXPERIMENTAL  PSYCHOLOGY 

Further,  we  have  neglected  the  influence  of  the  length  of 
the  standard  and  the  distance  between  the  two  lines. 

To  analyse  and  to  estimate  these  various  factors  is  the 
special  province  of  experimental  psychology.  In  so  doing, 
care  must  be  taken  that  practice,  fatigue,  and  interest  have 
the  same  play  in  differently  planned  investigations  ;  and  we 
must  assure  ourselves  that  the  differences,  met  with  under 
intentionally  altered  experimental  conditions,  are  significant 
and  not  accidental  (page  125). 

We  shall  proceed  on  the  supposition  that  these  sources 
of  danger  have  been  avoided.  Let  us  now  turn  to  a  con- 
sideration of  the  two  constant  errors  resulting  from  the 
position  of  the  variable  to  the  right  or  to  the  left  of  the 
standard  line.  If  el  denote  the  former,  and  e2  the  latter 
constant  error,  we  might  expect  that  the  two  errors  would 
be  equal.  As  a  matter  of  fact  they  are  usually  unequal. 
If  we  suppose  that  these  two  errors  (i.e.  the  errors  due 
to  the  rightward  or  leftward  position  of  the  variable 
relatively  to  the  standard)  are  of  equal  and  opposite 
value,  and  if  ±  q  denote  this  "  space  error/' 1  then  there  is 
in  addition  an  unknown  factor  Jc,  influencing  the  subject's 
determination.  That  is  to  say,  el  =  Jc  -f  q ;  e2  =  k  —  q. 
From  these  two  simultaneous  equations  the  values  of 
k  and  q  may  be  easily  found  in  terms  of  el  and  ez. 

The  "  time  error,"  due  to  the  relative  temporal  positions 
of  the  standard  and  variable  stimuli,  has  no  existence  in 
the  method  of  mean  error ;  for  here  the  standard  never 
follows,  but  always  precedes  or  accompanies  the  variable 
stimulus. 

The  other  factors,  to  which  we  have  drawn  attention, 
may  be  successively  investigated  in  like  manner  ;  until 
ultimately,  by  the  aid  of  introspection  which  is  afforded  by 
the  subject,  and  by  changes  in  experimental  conditions 
which  are  planned  by  the  observer,  all  the  psychologically 

1  It  is  usual  to  give  a  positive  or  negative  value  to  the  space  error,  as  the 
left-hand  object  appears  greater  or  less  than  the  right-hand. 


THE  PSYCHO-PHYSICAL  METHODS        205 

important  influences,  affecting  the  constant  error,  have  been 
analysed  and  estimated. 


THE  LIMITING  METHOD. 

This  method,  often  called  the  "  method  of  just  perceptible 
difference "  or  "  the  method  of  minimal  changes,"  has  a 
wider  range  of  utility  than  the  preceding.  In  this  method 
the  values  of  the  variable,  instead  of  being  changed  by  the 
subject,  are  fixed  and  prescribed  by  the  experimenter. 

Its  Simplest  Form. — The  subject  begins  with  a  standard 
stimulus,  S,  and  a  variable  stimulus,  F,  which  is  sufficiently 
different  from  the  standard  as  to  be  obviously  greater.  The 
experimenter  then  plies  the  subject  with  successively 
diminishing  values  of  the  variable  (e.g.  V—d,  V—  2d,  V—  3d, 
F-45),  each  given  in  conjunction  with  the  standard,  until 
finally  the  subject  judges  that  a  certain  variable  (e.g.  V-xS) 
and  the  standard  are  equal. 

A  series  of  variables,  thus  graded,  each  presented  once 
in  regular  descending  order  with  the  same  standard,  con- 
stitutes a  single  series  of  observations.  A  further  series 
must  now  be  obtained  by  reversing  the  order,  the  subject 
starting  with  a  variable  which  is  so  slightly  in  excess  of 
the  standard  that  they  appear  to  him  equal  to  one 
another,  the  experimenter  regularly  increasing  the  variable 
in  successive  pairs  of  presentations,  until  the  subject  at 
length  judges  that  the  variable  is  greater  than  the  standard. 

When  no  standard  presentation  is  used,  the  limiting 
method  becomes  applicable  to  the  determination  of  the 
absolute  threshold.  For  such  purpose,  of  course,  procedure 
is  only  available  by  descent  from  the  value  of  the  easily 
appreciable  stimulus  or  by  ascent  from  the  value  of  the 
unappreciable  stimulus. 

The  size  of  d  must  be  appropriate  to  the  conditions  of 
the  experiment.  If  it  be  too  large,  the  answers  become  too 
easy ;  if  it  be  too  small,  they  become  too  difficult.  In 


206  EXPERIMENTAL  PSYCHOLOGY 

either  case  the  subject's  interest  and  attention  are  liable  to 
flag. 

The  upper  differential  threshold  or  limen  obviously  lies 
between  the  last  (or  first — according  to  direction)  value 
of  the  variable,  which  just  allows  of  a  correct  judgment  of 
difference,  and  the  first  (or  last)  value  of  the  variable, 
which  just  produces  a  judgment  of  equality ;  and  we  may 
assume  that  it  lies  midway  between  the  two.  The  differ- 
ence between  the  mean  value  of  the  variable,  thus  derived 
from  a  number  of  such  series  of  observations,  and  the  value 
°f  the  standard  gives  the  upper  differential  threshold,!)11. 
va|ue  Of  ^  mean  variation  of  the  various  individual 
expresses  the  reliability  of  the  result. 

We  may  similarly  determine  the  value  of  the  lower 
differential  threshold,  D\  First  we  start  with  a  variable  Vv 
which  is  obviously  smaller  than  the  standard,1  adding  to 
its  increments  d,  2d,  35  .  .  .  ,  until  the  variable  and  the 
standard  appear  equal  to  the  subject ;  and  then  we  proceed 
in  reverse  order  from  apparent  equality  to  inequality  of  the 
presentations.  The  values  Du  and  Dl  will  slightly  differ 
from  one  another,  owing  to  the  operation  of  "Weber's 
law. 

[Procedure  ly  Complete  Descent  and  Ascent. — A  modifica- 
tion  of  the  limiting  method  enables  us  to  determine  Du  and 
Dl  in  the  same  investigation.  Starting  with  a  variable  V 
which  is  obviously  greater  than  the  standard  S,  we  may 
reduce  it  until  V  is  apparently  just  equal  to  S,  and  reduce 
it  still  further  until  V  is  just  less  than  £  Then  we  may 
increase  V  by  successive  stages  until  it  appears  to  be  just 
greater  than  S.  This  modification,  known  as  the  "pro- 
cedure by  complete  descent  and  ascent,"  gives  us  eight 
values  of  V..  Four  are  obtained  during  the  descent, 
namely,  (a)  that  V  which  is  for  the  last  time  greater  than 

1  F!  should  be  as  much  below,  as  V  was  above  the  threshold  ;  due 
allowance  being  made  for  differences  due  to  the  operation  of  Weber's 
law. 


THE  PSYCHO-PHYSICAL  METHODS        207 

S,  (8)  that  F  which  is  for  the  first  time  not  greater  than  S, 
(7)  that  V  which  is  for  the  last  time  not  less  than  $,  and  (5) 
that  V  which  is  for  the  first  time  less  than  $;  and  four 
corresponding  values  (<5P  y1}  /31?  a1)  are  successively  obtained 
during  the  ascent.] 

The  Effects  of  Adaptation  and  Expectation. — The  value  of 
thejipger  differential  threshold  obtained  by  diminishing  the 
variable  is  usually  lower  than  that  obtained  by  increasing  it. 
Similarly,  the  value  of  the  lower  differential  threshold,  is 
usually  lower  when  the  variable  is  increased  than  when  it  is 
lowered.  We  should  naturally  expect  this  to  result  from 
the  subject's  preceding  experience.  It  is  also  in  part  due 
to  a  kind  of  adaptation,  the  subject  inertly  tending  to 
continue  to  give  the  same  kind  of  replies  as  those  which 
he  has  already  given  for  the  immediately  previous  pairs  of 
presentations. 

This  tendency,  however,  may  be  more  or  less  out- 
weighed by  the  effect  of  expectation.  In  procedure  by 
regular  descent  or  ascent,  the  subject  cannot  fail  to  be 
influenced  by  the  knowledge  that  sooner  or  later  the  tenor 
of  his  answers  must  change,  that  at  some  point  his  judg- 
ments, which  for  example  have  hitherto  been  judgments  of 
excess,  must  change  into  those  of  equality.  On  the  other 
hand,  upon  certain  individuals  the  effect  of  such  expecta- 
tion may  be  diametrically  the  opposite.  They  may  so  far 
strive  to  prevent  their  anticipatory  attitude  from  in- 
fluencing the  turning-point,  that  thereby  the  latter  may 
even  be  postponed. 

Other  Modifications  of  the  Method. — Such  complications, 
unless  avoided,  seriously  affect  the  utility  of  the  limiting 
method.  We  have,  therefore,  to  consider  the  various 
devices  whereby  the  degree  of  the  subject's  foreknowledge 
may  be  reduced  as  far  as  possible.  [In  the  first  place,  the 
time  order  or  space  order  may  be  irregularly  varied.  That 
is  to  say,  in  any  single  series  of  observations,  sometimes  the 
standard  may  be  presented  before,  or  to  the  right,  or  after  or 


2o8  EXPERIMENTAL  PSYCHOLOGY 

to  the  left  of  the  accompanying  variable.  Again,  different 
grades  of  the  variable — V-s,  V-2e,  etc.,  instead  of  V-d, 
V-2d — may  be  occasionally  used  in  different  series  of 
observations.  Or,  thirdly,  the  descent  (or  ascent)  may  be 
begun  from  different  variables,  say  from  F±  5  or  from 
F±35  instead  of  from  V,  in  occasional  series.  These 
modifications  must,  however,  be  used  with  caution ;  other- 
wise the  remedies  bring  with  them  complications  greater 
than  those  which  they  are  intended  to  remove.  Thus  the 
four  time  and  space  orders,  in  which  the  variable  and 
standard  are  presented,  must  be  carefully  planned  so  that 
they  occur  with  equal  frequency  in  any  single  series ;  the 
number  and  size  of  the  grades  of  the  variable  must  be  the 
same  in  corresponding  ascending  and  descending  series ; 
and  the  number  of  series,  in  which  unusually  few  and 
unusually  many  grades  of  the  variable  are  employed,  must 
be  so  nearly  equal  that  their  opposite  effects  on  the  average 
of  the  total  series  may  counteract  one  another. 

When  several  series  of  observations  have  been  made 
with  one  time  or  space  order,  and  other  series  with  the 
opposite  time  or  space  order,  the  time  or  space  error  may 
be  easily  deduced  by  treating  the  respective  threshold 
values  in  precisely  the  same  manner  as  the  constant  errors 
were  treated  in  the  method  of  mean  error.] 

Various  other  modifications  of  the  limiting  method  have 
been  devised  to  ensure  unprejudiced  replies  on  the  part  of 
the  subject.  In  one  modification,  procedure  by  descent  and 
procedure  by  ascent  are  combined  so  that  successive  pairs  of 
stimuli,  within  each  single  series  of  observations,  alternately 
belong  to  one  or  other  of  the  two  procedures.  The  subject's 
answers  can  be  subsequently  sorted  out  and  treated  as  if 
they  had  been  given  in  two  separate  series,  one  of  descent 
and  the  other  of  ascent.  In  another  modification,  the  series 
of  variable  stimuli  are  presented,  each  with  the  standard  in 
irregular  order,  each  pair  occurring  once  in  every  series  of 
observations.  Here,  again,  the  data  may  be  appropriately 


THE  PSYCHO-PHYSICAL  METHODS        209 

sifted   and  the   upper  and   lower   thresholds   subsequently 
calculated. 

Another  device,  in  order  to  obtain  greater  freedom 
from  prejudice  on  the  part  of  the  subject,  consists  in  the 
occasional  presentation  of  a  variable  which  is  actually  equal 
to  the  accompanying  standard.  But  the  interpolation  of 
such  "catches"  has  an  undoubted  effect  on  the  general 
mental  attitude  of  the  observer,  and  it  is  difficult  to  ensure 
that  this  disturbing  effect  shall  be  constant  in  successive 
series  of  observation.  The  threshold  has  been  actually  found 
to  vary  with  the  number  of  catch  experiments  introduced. 

The  Method  of  Serial  Groups.  —  A  modification  of  the 
limiting  method  may  be  employed,  in  which  this  objection 
to  the  irregular  use  of  "catches"  does  not  apply.  It  has 
been  called  the  "method  of  serial  groups"  (exp.  103).  A 
given  value  of  the  variable,  F",  is  presented  with  a  standard, 
not  once  as  in  the  limiting  method,  but  several  (say  ten) 
times  before  it  is  changed  to  another  value.  These  ten 
pairs  of  stimuli  are  given  in  irregular  order  with  (say  ten) 
other  pairs  of  stimuli  in  which  the  variable  is  equal  to  the 
standard.  The  twenty  pairs  constitute  a  group.  If  the 
subject  acquits  himself,  satisfactorily  in  one  group,  he  passes 
to  a  second  group  of  twenty  pairs,  in  which  the  value  of  the 
variable,  now  reduced  to  F-3,  again  remains  constant,  and  in 
which  ten  catch  experiments  are  as  before  included.  Then 
the  experimenter  passes  to  a  third  group,  in  which  the  vari- 
able is  by  a  like  amount  further  reduced  to  F-25.  For  every 
group  the  relative  number  of  correct  replies  is  recorded. 
The  threshold  in  this  method  is  arbitrarily  fixed.  It  corre- 
sponds to  that  variable  which  yields  eighty  per  cent,  of 
right  answers.  A  similar  procedure  by  ascent  follows  that 
by  descent.  The  subject  is  previously  warned  that  every 
group  will  contain  catch  experiments  irregularly  distributed. 
In  calculating  the  threshold,  the  results  of  the  catch  experi- 
ments are  neglected;  but  they  are  otherwise  of  great 
interest,  psychologically. 
14 


210  EXPERIMENTAL  PSYCHOLOGY 

THE  CONSTANT  METHOD. 

The  constant  method  is  often  known  as  the  "Jmethod  of 
right  and  wrong  cases/?  In  this,  as  in  the  limiting  method, 
the  values  of  the  variable  are  prescribed  b^  the  experi- 
menter. But  the  number  of  grades  of  variables  is  usually 
much  less ;  they  need  not  form  a  regular  series  (although  it  is 
more  convenient  that  they  should) ;  the  order  of  presenta- 
tion is  irregular ;  and  any  given  variable  is  presented  not 
once  but  many  times  with  the  standard  in  single  series  of 
observations.  The  percentage  of  right  and  wrong  answers 
is  calculated  for  each  value  of  the  variable,  and  from  them 
an  attempt  is  made  to  arrive  at  the  limen  or  threshold 
(exp.  102).  (  The  threshold  is  deduced  from  that  value  of  the 
stimulus  or  of  the  stimulus  difference,  which  in  a  lopg 
series  of  trials  turns  out  to  be  as  often  appreciable  as 
inappreciable.^ 

Thus,  in  determining  the  spatial  threshold,  the  following 
percentages  of  answers  may  be  cited.1  The  answers  are 
judgments  that  two  points  are  touching  the  skin,  when 
it  is  simultaneously  touched  by  two  points  at  various 
distances  apart.  The  distances  are  expressed  in  Paris 
lines  (  =  2'25  mm.). 

Distance  of  points  .         .        .         .00 '5      11 -52      345        6 
Percentage  of "  two  point  "answers  30     10     14     40-^65     80     87     96     100 

Clearly  the  true  threshold,  D,  is  at  that  distance  which 
gives  an  equal  number  (50  per  cent.)  of  right  and  wrong 
answers.  In  the  above  investigation  its  value  lies  between 
1*5  and  2  lines.  Now,  if  Da  be  the  distance  at  which  a  per 
cent,  of  correct  answers  is  obtained,  and  Z>/3  be  the  distance 
at  which  j8  per  cent,  of  correct  answers  is  obtained,  and  if 

1  These  data  were  published  by  A.  Riecker  (Ztschr.f.  J3ioL,  1874,  x.  190), 
and  are  here  modified  for  purposes  of  exposition  according  to  Titchener 
(Experimental  Psychology,  New  York,  1905,  ii.  pt.  1,  pp.  92,  93,  and  pt.  2, 
p.  250). 


THE  PSYCHO-PHYSICAL  METHODS        211 

the  value  of  D  lies  above  Da  and  below  D/3,  it  can  be 
roughly  determined,  by  the  formula 
Da    £ 


to  have  the  value  of  1-7  lines. 

This  value,  however,  is  for  many  reasons  only  approxi- 
mate. In  the  first  place,  the  formula  assumes  that  the 
percentage  of  correct  judgments  increases  proportionately 
with  increase  of  the  distance  between  the  two  points.  But 
this  assumption  is  untrue;  the  percentage  increases  more 
rapidly  with  increase  from  small  to  larger  distances  than 
from  larger  to  still  larger  distances. 

In  the  second  place,  we  neglect  the  necessarily  different 
values  of  D  which  result  from  other  values  of  Da  and  D(3 
and  other  values  of  a  and  (3.  Clearly,  the  above  experiment 
might  have  been  so  planned  that  we  had  no  record  of  the 
percentage  of  right  answers  for  the  distance  of  2  Paris  lines 
between  the  two  points.  We  should  in  that  case  have 
inferred  that  the  spatial  threshold  lay  somewhere  between 
1-5  lines,  giving  40  per  cent.,  and  3  lines,  giving  80  per  cent. 
of  right  answers.  Applying  the  above  formula,  the  value  of 
the  threshold  now  becomes  T875  lines  instead  of  1'7  as 
before.  We  have  therefore  to  take  into  consideration  not  a 
single  pair  but  all  the  different  values  obtained,  paying  due 
weight  to  the  number  of  experiments  that  have  afforded 
these  several  values,  and  to  the  degree  of  scatter  of  the 
various  thresholds  from  which  the  mean  threshold  is 
derived.  These  conditions  can  be  satisfied  by  the  use  of 
complicated  formulae,  deduced  from  Gauss's  law  of  error 
(page  125),  which  lie  beyond  the  scope  of  an  elementary 
treatise.1 

The  threshold  may  be  roughly  determined  by  drawing 
a  frequency  curve  (fig.  6),  the  ordinates  representing  the 

1  A  mode  of  employing  the  constant  method  without  recourse  to  Gauss's 
formulae  has  been  recently  suggested  by  Spearman  (op.  cit.),  to  which  the 
more  advanced  student  may  be  referred. 


212  EXPERIMENTAL  PSYCHOLOGY 

various  observed  percentages  of  right  answers,  the  abscissae 
representing  (in  the  example  we  are  considering)  the 
distances  upon  the  skin  at  which  those  percentages  are 
respectively  given.  The  value  of  the  abscissa  at  the 
ordinate  value  of  50  may  be  then  determined  by  graphic 
measurement,  after  smoothing,  so  far  as  possible,  the 
irregularities  of  the  curve. 

[Reversals. — We  have  hitherto  assumed  that  the  percent- 
ages of  right  answers,  returned  at  the  various  distances 
between  the  two  points,  are  thoroughly  trustworthy.  But 
a  glance  at  the  curve,  which  we  have  drawn,  shows  us  that 
we  must  take  into  account  certain  so-called  "  reversals  "  of 

percentage  values.  We 
should  expect  the  percentage 
of  right  answers  to  increase 
more  rapidly  for  the  earlier 
(smaller)  than  for  the  latter 
(larger)  distances.  For  ex- 
ample, we  should  expect  the 
difference  between  the  per- 
centages of  right  answers 


FlG>  Q  returned   for   the    distances 

1  and  1*5  lines  to  be  greater 

than  for  the  distances  1*5  and  2  lines ;  and  so  it  is.  But 
this  relation  is  reversed  in  the  case  of  the  percentages  for 
3,  4,  and  5  lines,  and  again  for  0'5,  1,  and  1'5  lines.  In 
each  of  these  cases  the  increase  of  correct  answers  is  less 
from  the  first  to  second  than  it  is  from  the  second  to  third 
of  these  distances.  They  are  instances  of  what  has  been 
called  "  reversal  of  the  second  order." 

It  is  not  uncommon  even  to  find  a  "  reversal  of  the  first 
order."  That  is  to  say,  an  increase  of  the  distance  between 
the  two  points,  so  far  from  producing  an  absolute  increase, 
produces  an  absolute  decrease  in  the  percentage  of  right 
answers.  In  a  sense,  we  have  an  example  of  such  a  reversal 
in  the  present  series,  for  the  percentage  of  correct  judgments 


THE  PSYCHO-PHYSICAL  METHODS        213 

is  less  when  the  points  are  separated  by  0'5  lines  than  when 
they  are  not  separated  at  all,  i.e.  when  only  one  point  is 
presented. 

An  obvious  safeguard  against  these  reversals  consists  in 
increasing  the  number  of  observations  and  of  series  of 
observations.  But  beyond  certain  limits  this  becomes 
impracticable.  Moreover,  the  influence  of  practice  and 
fatigue  may  make  the  initial  and  terminal  results  in- 
comparable.] 

So  far,  for  simplicity's  sake,  we  have  been  considering 
the  constant  method  in  its  application  to  conditions  where 
merely  one  variable  stimulus  is  employed.  But  the  same 
method  has  been  still  more  widely  used  in  recording  the 
judgments  between  pairs  of  stimuli  (exp.  102),  or  between 
pairs  of  stimulus  differences  (exp.  122).  It  has  been  em- 
ployed, too,  with  most  valuable  results,  in  order  to  determine 
the  effect  of  prescribed  conditions  on  the  proportion  of  right 
and  wrong  answers  (exp.  121). 

Contrast  Effect. — In  all  these  applications  of  the  constant 
method,  an  important  cause  of  the  reversals,  to  which  we 
have  just  been  alluding,  lies  in  the  play  of  contrast  effects. 
As  the  various  pairs  of  variable  and  standard  are  presented 
in  irregular  order,  it  must  often  happen  (unless  the  experi- 
menter takes  special  precautions  to  avoid  it)  that  the 
presentation  of  a  large  difference  immediately  follows  or 
is  immediately  followed  by  the  presentation  of  a  small 
difference  to  the  subject.  Such  juxtaposition  of  extremes 
cannot  fail  unconsciously  to  produce  a  contrast  effect.  The 
difference  between  a  pair  of  stimuli,  which  the  subject  could 
correctly  distinguish  under  ordinary  circumstances,  may  no 
longer  be  detected,  if  it  be  immediately  preceded  by  one  or 
more  larger  and  very  obvious  differences. 

Side  Comparisons. — Moreover,  the  answers  may  at  any 
time  be  influenced  by  conscious  comparison  of  one  member 
of  a  pair  with  a  member  of  a  preceding  pair  of  stimuli. 
The  existence  of  these  "side  comparisons"  is  almost 


214  EXPERIMENTAL  PSYCHOLOGY 

invariably  revealed  by  adequate  introspection  during  such 
an  investigation. 

"  Doubtful "  and  "  Equal "  Answers. — In  describing  the 
constant  method,  we  have  hitherto  assumed  that  the  stimulus 
difference  is  judged  by  the  subject  to  lie  unquestionably 
either  in  one  direction  or  in  the  other ;  that  is  to  say,  we 
have  assumed  that  the  answers  can  easily  be  classified  as 
simply  right  or  wrong.  This,  however,  rarely  occurs  in 
actual  experiment.  For,  in  the  first  place,  the  subject  is  at 
any  time  liable  to  return  the  answer  "doubtful," — though 
along  with  increasing  practice  the  number  of  "doubtful" 
answers  very  much  diminishes.  And,  in  the  second  place, 
the  subject  will  often  return  the  answer  "  equal." 

The  question  arises,  How  are  we  to  deal  with  these 
"  doubtful "  and  "  equal  "  answers  ?  Although  to  some  extent 
they  differ  from  one  another,  that  difference  is  far  less  than 
their  difference  from  the  "  greater  "  or  "  smaller "  answers. 
Obviously,  their  place  is  somewhere  intermediate  between 
the  two  last. 

Some  psychologists  have  neglected  these  "  equal "  and 
"  doubtful "  answers  altogether  in  evaluating  their  results, 
some  have  forbidden  the  subject  ever  to  return  them,  while 
others  have  divided  their  number  equally,  adding  half  to  the 
right  and  half  to  the  wrong  answers.  The  first  procedure  is 
obviously  unscientific;  the  second  invites  the  subject  to 
abandon  his  desirable  attitude  of  conscientiousness;  the 
third  seems  the  only  justifiable  procedure  under  the  circum- 
stances. 

When  the  "equal"  (and  "doubtful")  answers  are  fre- 
quent enough  to  allow  of  their  being  grouped  together  and 
separated  from  the  "  greater  "  answers  on  the  one  hand,  and 
from  the  "  smaller  "  answers  on  the  other,  we  can  determine 
by  several  methods  the  average  value  of  the  magnitude  of 
the  variable  which,  when  presented  with  the  constant,  gives 
the  greatest  percentage  of  equal  answers.  Thus,  if  in  a  full 
series  of  experiments  stimulus  A  is  found  to  give  a  "  equal  " 


THE  PSYCHO-PHYSICAL  METHODS        215 

answers,  and  if  stimuli  B,  C,  D,  etc.,  similarly  give  b,  c,  d, 
etc.,  "  equal"  answers,  then  the  mean  value  of  the  required 
variable  can  be  deduced  from  the  expression 


Or  we  may  use  an  alternative  method  which  has  been 
described  as  a  combination  of  the  limiting  and  constant 
methods.  If  a  in  a  series  be  the,  smallest  value  of  the 
variable  stimulus  which  produces  the  answer  "  greater  "  and 
&  be  its  greatest  value  which  does  not  produce  the  answer 
"greater,"  and  if  c  be  the  smallest  value  which  does  not 
produce  the  answer  "  smaller  "  and  di  be  the  greatest  value 
which  does  not  produce  the  answer  "  smaller,"  then  a  mean 
can  be  derived  from  the  expression 


Other  methods  besides  these  two  here  mentioned  are  avail- 
able. But  into  these  and  into  a  discussion  of  their  com- 
parative utility  and  reliability  we  cannot  enter  here. 

Method  of  Equal-  appearing  Intervals.  —  The  method  of 
mean  error,  the  limiting  and  the  constant  methods  may  be 
also  used  to  determine  the  conditions  of  equality  (or  of  just 
appreciable  difference)  between  two  experiences,  which, 
instead  of  relating  to  two  stimuli  a  and  &,  relate  to  two  differ- 
ences between  the  pairs  of  stimuli  a  and  6,  c  and  d  (page  201). 
When  thus  applied,  the  psychological  procedure  sometimes 
receives  a  special  name.  It  is  known  as  the  "  method  of 
equal-appearing  intervals,"  or  the  "  method  of  mean  grada- 
tions." The  latter  expression,  however,  is  limited  to  the 
case  of  three  presentations,  a}  I,  c,  where  I  has  to  be  found 
which  appears  to  be  intermediate  in  value  between  a  and  c. 
It  is  strictly  inapplicable  to  the  case  when  that  difference 
between  c  and  a  variable  d  has  to  be  found,  which  appears 
equal  to  the  difference  between  a  and  5. 

In  point  of  fact,  there  is  no  single  method  of  equal- 
appearing  intervals.  It  is  merely  a  special  instance  of  the 


216  EXPERIMENTAL  PSYCHOLOGY 

application  of  the  method  of  mean  error,  the  limiting  method 
or  the  constant  method  to  a  particular  problem. 

The  Attitude  of  the  Subject. — Lastly,  we  have  to  consider 
the  attitude  of  the  subject  in  these  methods  of  experiment. 
He  may  be  allowed  to  give  his  replies  with  "  full  informa- 
tion," with  "  partial  information,"  or  "  without  information," 
according  to  the  purpose  of  the  investigation.  Thus  he  may 
approach  the  experiment  without  knowledge  either  of.  the 
direction  of  the  difference  between  the  stimuli  or  of  the 
space  order  or  time  order  which  the  experimenter  is  employ- 
ing. On  the  other  hand,  he  may  for  special  reasons  be 
allowed  the  foreknowledge  that  the  standard  is  smaller  or 
larger  than  the  variable,  or  that  it  lies  to  the  right  or  left 
of,  or  is  presented  to  him  before  or  after,  the  variable.  Or 
he  may  proceed  with  partial  information,  the  actual  con- 
dition of  the  presentation  being  told  him  after  each  answer. 
A  comparison  of  the  different  results  thus  obtained  is  of 
considerable  psychological  interest. 

According  to  the  purpose  of  the  experiment,  the  subject 
may  be  required  to  give  his  answers  always  in  terms  (i.) 
of  the  first  or  of  the  second  stimulus  or  pair  of  stimuli,  (ii.) 
of  the  right-hand  or  of  the  left-hand  stimulus  or  pair  of 
stimuli,  or  (iii.)  of  the  variable  or  of  the  standard.  On 
the  other  hand,  (iv.)  he  may  be  left  absolutely  free  to 
express  himself  as  he  pleases,  e.g.  after  successively  lifting  a 
pair  of  weights,  he  may  say  "right-hand  weight  heavier," 
or  "second  weight  lighter,"  and  so  on.  As  we  shall  see 
(page  271),  such  liberty  of  behaviour  throws  light  on  the 
mental  attitude  involved  in  any  particular  judgment. 

The  possibly  disturbing  effects  of  making  introspective 
records  during  experiments  have  also  to  be  reckoned  with. 
That  is  to  say,  series  of  observations  with  introspection 
should  be  compared  with  those  unaccompanied  by  intro- 
spection. 

Correspondence  between  Results  of  Limiting  and  Constant 
Methods. — The  play  of  complicating  psychological  factors  is, 


THE  PSYCHO-PHYSICAL  METHODS        217 

as  we  have  seen,  so  different  in  the  three  psycho-physical 
methods  that  a  priori  we  should  not  expect  an  exact  corre- 
spondence in  the  results  to  which  they  lead.  The  method  of 
mean  error,  in  which  the  subject  is  free  to  alter  the  variable 
presentation,  is  obviously  incomparable  with  the  limiting 
and  constant  methods,  in  which  the  value  of  the  variable  is 
each  time  prescribed  by  the  experimenter.  The  two  last 
methods,  however,  may  be  made  to  approach  one  another,  if 
that  form  of  the  limiting  method  is  employed,  in  which  the 
variables  are  plied  haphazard  instead  of  in  orderly  descent 
or  ascent. 

BIBLIOGRAPHY. 

G.  T.  Fechner,  Elemente  d.  Psychophysik,  Leipzig,  1860 ;  Revision  d. 
Hauptpunkte  d.  Psychophysik,  Leipzig,  1882.  G.  E.  Mliller,  "Die  Gesichts- 
punkte  u.  d.  Tatsachen  d.  psychophys.  Methodcn,"  in  Asher  and  Spiro's 
Ergebnisse  d.  PhysioL,  Wiesbaden,  1903,  ii.  Abth.  2,  267  (also  published 
separately).  E.  B.  Holt,  "The  Classification  of  Psychophysic  Methods," 
Psychol.  Rev.,  1904,  xi.  343.  E.  B.  Titchener,  Experimental  Psychology, 
New  York,  1905,  ii.  parts  1,  2.  C.  Spearman,  "The  Method  of  'Right 
and  Wrong  Cases '  ('Constant  Stimuli')  without  Gauss's  Formula, "  Brit. 
Journ.  of  Psychol.,  1908,  ii.  227. 


CHAPTER    XVI 
ON  WEIGHT 

The  Effects  of  Tactual  and  Motor  Anaesthesia. — We  have 
already  observed  (page  68)  that  an  individual,  deprived  of 
the  normal  tactual  and  motor  sensations  in  any  part  of  his 
body,  is  unable  to  move  the  part  with  accustomed  certainty 
and  precision,  or  to  appreciate  the  position  of  the  part  or 
the  event  of  its  active  or  passive  movement,  when  his  eyes 
are  closed.  He  believes  that  he  has  moved  the  part,  when 
it  has  been  so  held  as  to  be  immovable.  He  believes  that 
the  part  is  stationary,  when  it  has  really  moved. 

Conjoined  with  these  defects  is  the  inability  to  estimate 
weight.  When  a  limb  is  totally  anaesthetic,  heavy  and  light 
objects  of  like  outward  appearance  appear  of  equal  weight. 
So  long  as  motor  sensation  is  intact,  the  power  of  estimating 
the  weight  of  lifted  objects  is  preserved.  It  is  retained, 
even  when  the  skin  has  been  rendered  anaesthetic. 

Behaviour  in  Estimating  Weight. — In  judging  the  weight 
of  objects  merely  by  their  contact  with  our  body  (as  when 
an  object  presses  upon  the  unsupported  hand),  our  estimate 
is  based  partly,  perhaps,  on  the  cutaneous  sensations,  but 
principally  on  the  motor_sensations  of  deep  pressure  and 
tension  which  the  object  evokes.  Under  such  conditions, 
our  judgments  of  weight  are  very  largely  influenced  by  the 
rate  of  application  of  the  object  to  our  body. 

In  ordinary  life,  however,  we  are  in  the  habit  of  estimating 
the  weight  of  objects  by  raising  or  lowering  them,  or  by  raising 

and  lowering  them.     To  our  experiences  of  tension  and  of 

218 


WEIGHT  219 

pressure  we  thereby  add  those  of  movement.  We  are  able 
to  note  the  speed  with  which  and  the  height  to  which  the 
object  rises,  and  the  time  that  elapses  between  the  determina- 
tion to  lift  the  object  and  its  actual  ascent.  All  these  factors 
are  of  the  greatest  importance  in  influencing  our  estimate  of 
weight. 

Influence  of  the  Speed  of  Lifting  Weights. — The  influence 
of  the  speed  with  which  lifted  objects  rise  from  the  ground, 
on  our  judgment  of  their  weight  is  open  to  experimental 
investigation.  An  electric  circuit,  connected  with  a  chrono- 
scope  (see  exp.  88),  is  so  arranged  that  the  current  is 
interrupted  directly  the  object  leaves  the  ground,  and  is 
restored  when  the  object,  during  its  ascent,  makes  a  second 
contact  at  a  known  height  from  the  ground.  Experiments, 
conducted  on  these  lines,  have  shown  the  great  import- 
ance of  the  rate  of  movement  upon  our  estimation  of 
the  weight  of  a  lifted  object.  ( Other  things  being  equal,  the 
more  rapidly  a  lifted  object  ascends,  the  lighter  it  appears 
to  be.  } 

This  influence  of  the  rate  of  movement  persists,  when 
the  eyes  are  closed  and  when  the  skin  has  been  rendered 
ansesthetic.  It  disappears  in  the  case  of  patients  suffering 
from  locomotor  ataxia,  a  disease  in  which  motor  sensation  is 
much  impaired.  Its  dependence  upon  the  sensations  of  the 
motor  apparatus  is,  therefore,  unquestionable. 

When  two  objects  are  simultaneously  lifted,  one  by  each 
hand,  our  comparison  of  their  weights  appears  to  be  affected 
by  the  difference  in  the  time  taken  to  set  each  of  the  objects 
in  motion.  (If  two  objects  of  actually  equal  weight  be  lifted 
with  apparently  equal  expenditure  of  force,  and  if  one  object 
begin  to  rise  before  the  other,  it  seems  that  that  object  tends 
to  be  judged  the  lighter.  J 

Tactual  and  Visual  Influences. — In  proceeding  to  lift  an 
object,  we  are  also  guided  by  our  tactual  or  visual  perception 
of  it.  Experience  has  taught  us  that,  when  a  large  object 
presses  on  the  skin,  or  when  a  large  object  is  grasped  or  seen, 


220  EXPERIMENTAL  PSYCHOLOGY 

the  amount  of  muscular  effort,  requisite  to  lift  it,  is  greater 
than  in  the  case  of  a  smaller  object. 

The  Size-  Weight  Illusion.  —  Consequently,  when  two 
objects  of  unlike  size  and  of  equal  weight  are  lifted,  the  fact 
that  they  are  felt  or  seen  to  be  of  unlike  size  irresistibly 
determines  a  difference  in  the  strength  of  muscular  con- 
traction put  forth  in  order  to  lift  each  of  them.  The  larger 
object  rises  more  rapidly  than  the  smaller  one  and  is  for 
this  reason  judged  to  be  the  lighter  of  the  two,  whereas  the 
weight  of  the  two  objects  is  actually  the  same.  This  cause 
of  error  is  commonly  called  the  "  size-weightjllusion."  By 
reason  of  it,  a  pound  of  feathers  appears  lighter  than  a 
pound  of  lead  (exp.  102). 

The  size-weight  illusion  may  be  investigated  and  measured 
under  various  conditions.  Thus  the  subject  may  be  per- 
mitted both  to  see  the  weights  and  to  grasp  them ;  or  he 
may  be  required  to  grasp  them  blindfold ;  or  with  open  eyes 
he  may  be  allowed  to  raise  the  weights  by  grasping  handles 
of  equal  size  attached  to  them.  The  illusion  is  present  in 
all  three  cases,  but  is  strongest  in  the  first,  where  prehensile 
and  visual  influences  are  conjoined.  It  varies  widely  in 
different  individuals,  and  is,  as  we  might  expect,  more 
marked  in  adults  than  in  children.  C  Indeed,  in  very  young 
children,  the  apparent  difference  in  weight  of  the  two  objects 
is  said  to  be  absent,  or  even  reversed,  the  larger  object  then 
appearing  the  heavier.) 

We  have  described  the  difference  in  our  mode  of  lifting 
the  objects  in  the  size-weight  illusion  as  irresistible.  That 
is  to  say,  it  persists,  in  some  considerable  degree,  even  after 
we  have  been  made  aware  of  the  illusion.  We  can  therefore 
hardly  ascribe  the  illusion  to  "  suggestion,"  in  the  ordinary 
sense  of  the  word.  "  Unconscious  inference "  would  be  a 
better  term.  The  amount  of  muscular  effort,  put  forth  in 
lifting  an  object,  is  unconsciously  inferred  from  and  irresist- 
ibly determined  by  its  size. 

Even  when  two  objects  are  of  like  size  and  weight,  the 


WEIGHT  221 

fact  that  the  one  is  seen  to  be  of  light  and  the  other  of 
heavy  material  influences  the  amount  of  muscular  contrac- 
tion put  forth.  For  this  reason  a  solid  cube  of  wood  appears 
to  be  heavier  than  a  hollow  leaden  cube  of  like  size  and 
weight. 

Motor  Attunement. — We  have  now  to  discuss  another 
condition  which  unconsciously  influences  the  amount  of 
muscular  effort  put  forth, — a  condition  which  we  may  term 
"  motor  attunement."  Experiments  have  shown  that,  if  two 
objects  of  equal  size  but  of  different  weight  (e.g.  two  like 
canisters  weighing  676  and  2476  grams)  be  alternately  lifted, 
say  thirty  times,  the  lighter  say  by  the  right  hand  and  then 
the  heavier  by  the  left  hand,  with  equal  speed  and  to  an 
equal  height,  a  state  of  motor  attunement  is  ultimately 
established.  In  this  condition,  an  object  lifted  by  the  left 
hand  appears  lighter  than  an  equal  or  evenjighter  object 
then  lifted  by  the  right  hand.  Thus,  when  in  the  above 
experiment  one  or  other  of  a  series  of  weights,  varying  from 
826  to  926  grams,  is  substituted  for  the  heavier  of  the  two 
weights,  it  is  judged  by  the  subject  to  be  lighter  than  the 
really  lighter  weight  of  676  grams. 

Evidently  we  have  at  hand  a  ready  explanation  of  this 
error  of  judgment,  if  the  effect  of  repeatedly  lifting  a  lighter, 
followed  by  a  much  heavier,  object  is  to  establish  an  attune- 
ment or  "  set "  of  the  system,  which  manifests  itself  in  a 
tendency  to  lift  the  second  of  any  subsequent  pair  of  weights 
with  a  greater  expenditure  of  force  than  is  given  to  the  first. 
That  second  weight,  if  it  be  no  longer  much  heavier  than  the 
first,  must  rise  with  greater  speed  than  the  first  and  accord- 
ingly appear  the  lighter.  We  have  undoubted  evidence  that 
this  more  rapid  rise  of  the  second  lifted  weight  actually 
occurs,  and  that  it  is  the  cause  of  the  erroneous  judgment. 

When  attunement  has  been  in  this  way  established,  our 
judgment  may,  at  the  same  time,  be  influenced  in  the 
opposite  direction  by  the  more  intense  sensations  of  pressure 
and  tension  which  the  second,  actually  heavier,  weight 


222  EXPERIMENTAL  PSYCHOLOGY 

evokes ;  or  our  judgment  may  be  embarrassed  owing  to  the 
unexpected  rapidity  with  which  this  weight  rises.  There 
are,  therefore,  tendencies  at  work  either  to  mitigate  the 
error  or  to  increase  the  uncertainty  of  our  judgment. 

In  the  above  -  mentioned  experiment,  attunement  is 
effected  by  alternately  raising  two  weights,  one  in  either 
hand.  It  may  likewise  be  effected  when  the  two  weights 
are  consecutively  lifted  by  the  same  hand,  or  when  a  series 
of  lifts  of  light  weights  precedes  the  lifting  of  a  heavy 
weight.  In  the  above  experiment,  too,  the  order  in  which 
the  weights  are  lifted  is  "  light,  heavy."  That  is  to  say,  first 
a  light  weight  is  lifted,  and  then  a  heavy  weight  is  lifted ; 
whereupon  the  subject  is  asked  to  compare  the  weights. 
Attunement  can  also  be  effected  in  the  reverse  direction, 
namely,  "  heavy,  light."  It  is,  however,  usually  less  marked 
and  may  even  be  reversed,  owing  to  some  counteracting 
influence  of  the  time  error  (page  204). 

A  motor  attunement,  thus  acquired,  disappears  in  course 
of  time,  at  first  rapidly,  later  more  slowly.  In  certain 
experiments,  traces,  more  or  less  evident,  of  such  a  motor 
attunement  have  been  found  even  twenty-four  hours  after 
its  acquisition. 

The  Possibility  of  Transference  of  Motor  Attunement. — 
The  following  will  serve  to  exemplify  an  investigation, 
designed  to  determine  whether  motor  attunement,  acquired 
on  one  side  of  the  body,  has  any  effect  upon  the  correspond- 
ing movements  of  the  opposite  side.  A  series  of  pairs  of 
weights  (of  like  size  and  appearance)  are  lifted  consecutively 
by  one  hand.  One  of  the  members  of  each  pair  is  a  constant 
weight  of  500  grams  ;  the  other  is  a  variable  weight  of  450, 
500,  550,  600,  650,  or  700  grams.  The  constant  weight 
always  lies  to  the  right  of  the  variable,  and  is  always  lifted 
first.  The  two  weights  are  raised  and  lowered  through  an 
equal  distance.  A  metronome,  beating  eighty-four  strokes 
to  the  minute,  fixes  a  constant  speed  of  lifting.  The  first 
weight  of  a  pair  is  raised  at  the  first  stroke  of  the  metronome 


WEIGHT  223 

and  is  lowered  at  the  second  ;  the  second  weight  is  raised  at 
the  third  stroke  of  the  metronome  and  is  lowered  at  the 
fourth.  The  subject's  judgment  as  to  the  difference  or 
equality  in  the  two  weights  is  then  recorded.  Eighteen  lifts 
of  the  six  pairs,  presented  in  carefully  designed  order,  con- 
stitute what  we  may  term  the  "  pre-attunement  experiments." 
After  a  pause  of  two  minutes,  they  are  followed  by  the 
"  attunement  experiments  "  ;  in  which  a  series  of  sixty  pairs 
of  lifts  is  made,  the  pairs  being  a  weight  of  500  grams  lying 
to  the  right  and  lifted  first,  and  a  weight  of  2260  grams  (of 
course,  of  like  size)  lying  to  the  left  and  lifted  second. 
These  sixty  pairs  are  divided  into  six  groups,  each  group 
being  separated  by  an  interval  of  forty-five  seconds  from  the 
other. 

Finally,  after  another  pause  of  two  minutes,  come  the 
"  post-attunement  experiments,"  which  are  an  exact  repeti- 
tion of  the  pre-attunement  experiments,  and  are  to  be 
compared  with  them. 

These  eighteen  pairs  of  lifts  in  the  pre-  and  in  the 
post-attunement  experiments,  together  with  the  sixty  pairs 
of  lifts  in  the  attunement  experiments,  constitute  one  day's 
research.  The  whole  investigation  lasts  twenty  days,  which 
are  divided  into  five  four-day  groups.  On  the  first  day  of 
each  four- day  group,  the  pre-attunement,  the  "  habituation," 
and  the  post-attunement  experiments  are  all  performed  by 
the  left  arm,  on  the  third  day  they  are  all  performed  by  the 
right  arm ;  on  the  second  day  the  attunement  experiments 
are  performed  by  the  left  arm,  while  the  pre-  and  post- 
attunement  experiments  are  performed  by  the  right  arm ;  on 
the  fourth  day  the  attunement  experiments  are  performed 
by  the  right  arm,  the  pre-  and  post-attunement  experiments 
by  the  left. 

The  subject  is  required  to  give  his  answers  in  terms  of 
the  second  weight,  according  as  it  appears  to  him  heavier 
or  lighter  than,  or  indistinguishable  from,  the  first  of  each 
pair.  The  twenty  days'  judgments  are  here  tabulated, 


224 


EXPERIMENTAL  PSYCHOLOGY 


according  as  the  corresponding  answers  are  "  lighter  "  (I), 
"  indistinguishable  "  (i),  and  "  heavier  "  (h) : — 


Pre-Attuneiuent. 

Post-Attunement. 

Order  of 

Q'J  - 

Four-  Day 
Groups. 

Side  of 
Body. 

I 

i 

ft 

"Attuned." 

Side  of 
Body. 

I 

t 

h 

First  day 

left 

33 

20 

37 

left 

left 

47 

27 

16 

Second  ,, 

right 

28 

23 

39 

left 

right 

27 

26 

37 

Third    ,, 

right 

27 

26 

37 

right 

right 

46 

29 

15 

Fourth,, 

left 

37 

22 

31 

right 

left 

32 

28 

30 

We  see  that  it  is  only  on  the  first  and  third  days  that 
the  effect  of  the  attunement  experiments  is  manifest ;  the 
second  weight  appears  lighter  on  these  days.  There  is  thus 
no  evidence  of  extension  of  the  attunement  effect  to  the 
opposite  side  of  the  body. 

This  investigation  was  followed  by  others  in  which  the 
paired  weights  of  the  attunement  experiments,  instead  of 
being  lifted  in  "  light,  heavy "  sequence,  were  lifted  in 
"  heavy,  light "  sequence ;  and  by  others  in  which  the  two 
weights  of  the  attunement  experiments  were  equal.  But 
into  these  and  other  experimental  modifications  we  cannot 
enter  here.  Their  general  result  has  been  to  confirm  the 
conclusion  that  the  effects  of  attunement  are  confined  to 
those  muscles  on  which  the  attunement  experiments  have 
been  performed. 

The  Comparison  of  Motor  Attunement  with  the  Effects  of 
Stimulating  the  Visual  Cortex. — A  close  analogy  to  this 
condition  of  motor  attunement  is  to  be  found  in  the  altered 
effects  of  unilateral  stimulation  of  the  visual  cortex  in  the 
monkey,  when  this  unilateral  stimulation  has  been  preceded 
by  a  period  of  bilateral  stimulation  of  the  cortex.  Under 


WEIGHT  225 

ordinary  conditions,  unilateral  stimulation  of  the  proper 
zone  of  the  visual  cortex  produces  movement  of  both  eyes 
towards  the  opposite  side  of  the  body;  whereas,  during 
bilateral  stimulation,  the  eyes  assume  the  primary  or  a 
slightly  convergent  position.  But  it  has  been  shown  that 
after  a  period  of  bilateral  excitation,  subsequent  unilateral 
excitation  frequently  produces  a  bilateral,  not  a  unilateral 
effect.  Instead  of  moving  to  one  side,  the  two  eyes  fixate 
an  imaginary  object  midway  before  them. 

Now  we  have  experimental  evidence  that  the  movement 
of  the  eyes  to  one  side,  usually  produced  by  unilateral 
cortical  stimulation,  is  not  effected  merely  by  contraction  of 
the  appropriate  oculo-motor  muscles.  If  those  nerves,  which 
supply  solely  this  group  of  contracting  muscles,  be  divided, 
the  eye  still  behaves  as  before  to  cortical  stimulation.  This 
shows  that  normally  the  contraction  of  one  group  of  ocular 
muscles  is  accompanied  by  active  relaxation  or  inhibition  of 
the  opposing  group  of  muscles. 

On  the  other  hand,  during  bilateral  cortical  stimulation, 
both  groups  of  muscle  are  contracted.  Consequently,  it 
may  be  said  of  the  above  experiments  that  a  preliminary 
series  of  bilateral  stimulations  is  able  to  convert  cortical 
impulses,  which  would  normally  have  had  an  inhibitory 
effect,  into  impulses  which  have  an  excitatory  effect,  on  the 
motor  nuclei  of  the  oculo-motor  muscles.  Apparently  the 
synapses,  between  the  nuclear  and  the  cortical  neurones 
(page  74),  have  acquired  a  definite  setting  or  atturie- 
ment,  owing  to  their  having  been  previously  played  upon 
in  a  definite  manner. 

A  similar  attunement  may  be  supposed  to  have  occurred 
in  the  weight-lifting  experiments  which  we  have  had  under 
.consideration.  It  is  not,  as  before,  a  change  from  inhibition 
to  excitation,  but  a  change  in  the  intensity  of  excitation. 
Owing  to  this  acquired  setting  or  attunement,  the  volitional 
impulses  that  play  upon  the  cortical  arm  centre  no  longer 
have  their  normal  effect.  The  impulses,  which  descend 


226  EXPERIMENTAL  PSYCHOLOGY 

from  the  cortex  to  the  spinal  motor  nuclei,  are  now  either 
more  powerful  or  less  powerful  than  usual,  according  to  the 
kind  of  attunement  acquired. 

The  Comparison-  of  Motor  Attunement  with  the  Association 
of  Ideas.  —  In  several  respects,  this  condition  of  motor 
attunement  bears  a  striking  analogy  to  the  establishment 
of  association  between  two  ideas.  The  tendency  to  a  certain 
sequence  of  movement,  weak  following  or  followed  by 
strong,  may  be  acquired  by  weight-lifting,  just  as  the 
tendency  to  a  certain  sequence  of  reproductions  is  acquired 
by  learning.  This  analogy  between  motor  attunement  and 
the  association  of  ideas  is  borne  out  in  the  following  experi- 
ments. 

The  effects  of  one  motor  attunement  may  be  appar- 
ently "  counteracted  "  by  another  attunement  subsequently 
acquired.  If,  for  instance,  a  series  of  lifts  of  "  light,  heavy  " 
pairs  of  weights  (e.g.  those  of  the  attunement  experiments 
described  on  page  221)  be  followed  by  a  series  of  lifts  of 
"heavy,  light"  pairs,  an  apparent  "counteraction"  of  the 
first  attunement  is  readily  obtainable.  This  counteraction, 
however,  occurs,  even  when  a  series  of  "  equal  weight "  pairs 
is  used  in  place  of  the  "  heavy,  light "  series.  There  is, 
therefore,  no  true  counteraction ;  inasmuch  as  the  first 
attunement  disappears  when  the  second  is  neither  equal 
nor  opposite  to  it. 

We  should  be  wrong,  then,  in  supposing  that  the  cortex 
returns  to  its  primary  neutral  condition  when  one  attune- 
ment is  apparently  counteracted  by  another.  We  must 
suppose  that  the  two  attunements  neutralise  one  another 
by  inhibition,  rather  than  by  annihilation.  That  such  is 
the  case  is  indeed  proved  by  the  fact  that  the  older  attune- 
ment— the  attunement  which  is  acquired  first — outlasts 
the  younger.  It  tends  to  return  in  course  of  time,  although 
at  first  prevented  by  the  inhibitory  action  of  the  younger 
attunement.  Precisely  analogous  conditions  have  been 
experimentally  shown  to  govern  the  relative  strength  of 


WEIGHT  227 

the  associations  between  syllables,  a-b  and  a-c.  They 
react  on  one  another  by  inhibition,  and,  other  things  being 
equal,  the  older  association  outlasts  the  younger. 

Moreover,  it  is  found  that  the  longer  the  time  over 
which  a  given  amount  of  practice  (i.e.  a  given  number  of 
pairs  of  lifts)  in  acquiring  a  motor  attunement  is  distributed, 
the  stronger  will  that  attunement,  after  not  too  short  a 
pause,  prove.  Here  again  a  striking  analogy  will  be  found 
to  the  relation  between  the  strength  of  an  association  and 
the  distribution  of  the  repetitions  whereby  it  has  been 
acquired  (page  171). 

The  Sense  of  Effort. — So  far  we  have  without  question 
assumed  that  the  effort  of  lifting,  on  the  appreciation 
of  which  our  judgments  of  weight  are  so  dependent,  is 
determined  by  the  peripherally  derived  sensations  of  tension 
and  movement.  We  have  now  to  consider  a  totally  different 
view,  namely,  that  the  efferent  impulses,  discharged  from 
the  motor  cortical  areas,  give  rise,  at  the  moment  of  their 
discharge,  to  an  experience  (or  "  sensation  ")  of  effort,  and 
that  by  the  intensity  of  this  experience  we  are  informed 
of  the  force  of  the  muscular  contraction  which  we  are 
employing,  and  hence  of  the  weight  of  the  object  that  we 
are  lifting. 

At  one  time,  these  were  rival  views  and  excited  con- 
siderable controversy,  but  at  the  present  day  no  one  denies 
the  importance  of  motor  sensations.  The  evidence  of 
disease  and  experiment,  already  quoted,  is  now  too  strong 
to  be  neglected  even  by  the  strongest  partisan  of  the 
existence  of  the  central  "sense"  of  effort.  The  only 
question  deserving  of  consideration  is  whether  such  a  sense 
exists  in  us,  in  addition  to  kinsesthesis.  If  we  admit  it,  we 
have  at  the  same  time  to  admit  an  essential  difference 
between  active  and  passive  movements,  between  which  we 
have  already  noticed  a  close  resemblance  (page  69).  Never- 
theless, it  is  quite  conceivable  that  there  may  be  difficulties 
in  the  way,  the  solution  of  which  is  only  possible  by  an 


228  EXPERIMENTAL  PSYCHOLOGY 

assumption  that  we  have  a  central  sense  of  effort.  We 
proceed,  therefore,  to  discuss  two  such  difficulties  that  have 
been  brought  forward. 

Discussion  of  two  Difficulties. — In  cases  of  paralysis  of  a 
limb,  the  subject  can  make  an  effort  to  move  the  immobile 
limb,  and  he  actually  feels  the  effort  which  he  has  put 
forth.  But  because  kinaesthetic  sensations  are  no  longer 
obtainable  from  the  paralysed  limb,  we  need  not  refer  this 
sense  of  effort  to  a  central  origin.  Any  attempt  at  move- 
ment is  never  confined  to  a  single  muscle  or  limb,  but 
always  involves  other  parts  of  the  body.  The  muscles  of  the 
opposite  side  of  the  body  or  of  the  respiratory  system  are 
commonly  involved;  when,  for  example,  we  endeavour  to 
lift  a  weight,  the  glottis  closes,  the  abdominal  muscles  and 
diaphragm  contract.  Moreover,  many  movements  demand 
or  are  accompanied  by  the  fixation  of  other  parts  of  the 
body.  That  is  to  say,  certain  muscles  must  at  the  same 
time  be  thrown  into  contraction  to  prevent  movement ;.  and 
these  may  occasion  the  sensations  of  effort.  We  pass  on  to 
meet  the  second  difficulty. 

In  a  case  of  partial  or  complete  paralysis  of  the  external 
rectus  muscle  of  one, (say,  the  right)  eye,  the  subject  is 
unable  to  move  the  eye  to  the  outer  (right)  limit  of  its 
normal  range  of  movement.  If,  with  the  normal  (left)  eye 
closed,  he  endeavours  to  turn  the  eyes  to  the  extreme  right, 
instead  of  the  right  eye  so  turning,  external  objects  in  its 
field  appear  to  be  moving  to  the  side.  This  apparent 
movement  of  the  visual  field,  during  the  attempted  con- 
traction of  a  paralysed  muscle,  has  been  ascribed  to  the 
central  sense  of  effort.  It  has  been  supposed  that  the  effort 
leads  the  subject  to  believe  that  he  has  actually  made  the 
movement  which  he  would  under  normal  conditions  have 
made,  if  objects  were  passing  before  his  gaze ;  whereupon  he 
unconsciously  infers  that  the  objects  in  his  visual  field  are 
moving. 

But  this   explanation   neglects  the  movements   of   the 


WEIGHT  229 

healthy  closed  eye.  While  the  right  external  rectus  is 
paralysed,  the  left  eye  has  necessarily  been  moving  to  the 
right,  and  it  is  in  the  highest  degree  probable  that  the 
apparent  movement  of  the  field  of  the  right  eye  is  due  to 
confusion  between  the  experiences  of  the  two  eyes,  always 
so  closely  associated  with  one  another  (cf.  exp.  127).  The 
subject  believes  that  the  right  eye,  whereas  in  reality  the 
left  eye,  has  moved  to  the  right.  The  explanation  also 
neglects  the  effects  due  to  the  relaxation  of  the  internal 
rectus  in  the  right  eye  (page  225). 

When  both  eyes  are  open,  either  the  illusion  does  not 
occur,  or  there  is  double  vision  (diplopia)  and  the  illusion  is 
confined  to  the  paralysed  eye.  That  is  to  say,  in  the  one 
case  the  subject  has  come  to  neglect  the  visual  experiences 
of  his  paralysed  eye ;  in  the  other  the  motor  sensations  of 
the  left  eye  serve  to  interpret  in  different  ways  for  each  eye 
the  now  independent  experiences  of  the  retinae.  Again, 
then,  there  is  not  sufficient  cause  to  assume  the  existence  of 
central  sensations  of  effort. 

Other  Difficulties. — Nor  would  it  be  easy  to  comprehend 
the  physiological  basis  of  such  central  experiences  of  effort, 
even  if,  as  is  far  from  being  the  case,  the  necessity  for  their 
existence  had  been  psychologically  proved.  We  should 
have  to  assume  that  the  motor  nervous  impulse,  at  the 
moment  of  its  discharge  from  the  cortex,  was  directed 
towards  some  cortical  sensory  centre.  Now,  whether,  as  seems 
most  probable,  the  sensory  fibres  of  the  motor  apparatus 
terminate  in  special  kinsesthetic  areas  of  the  cortex,  or 
whether,  as  others  believe,  they  communicate  directly  and 
solely  with  the  cortical  motor  areas,  in  any  case  there  is 
unquestionably  a  passage  of  nervous  impulses  from  the 
sensory  towards  the  motor  neurones ;  and,  so  far  as  we  know, 
.the  direction  of  impulses  across  a  synapse  is  irreversible. 

We  should  expect  that,  if  such  backward  discharges 
were  possible,  they  would  not  evoke  "  sensations  of  effort," 
but  that  they  would  rather  revive  images  of  earlier,  or 


230  EXPERIMENTAL  PSYCHOLOGY 

produce  ideas  of  the  coming,  movements.  We  know, 
however,  that  the  true  order  of  events  is  precisely  the 
opposite.  It  is  in  the  attention  to  images  of  the  anticipated 
movement,  and  in  the  passage  of  these  images  into  move- 
ment, that  the  volitional  execution  of  unpractised  move- 
ments consists. 

We  may  conclude,  then,  that  what  effort  of  central  origin 
there  is  in  movement  is  of  the  same  nature  as  the  effort 
which  distinguishes  active  from  passive  movement,  and 
characterises  all  higher  forms  of  mental  activity,  for  example, 
reasoning  or  imagination.  It  is  the  effort  inherent  in  every 
conative  process. 

BIBLIOGRAPHY. 

G.  E.  Miiller  u.  F.  Schumann,  "  Ober  d.  psychol.  Grundlageu  d. 
Vergleichung  gehobener  Gewicbte,"  Arch.f.  d.  ges.  Physiol.,  1889,  xlv.  37. 
A.  D.  Waller,  "The  Sense  of  Effort,"  Brain,  1891,  xiv.  179.  S.  Exner, 
Kntwurf  zu  einer  pliysiol.  Erklarung  d.  psydvsclicn  Erschcinungcn,  Wien, 
1894,  i.  170.  C.  E.  Seashore,  "  Measurements  of  Illusions  and  Hallucina- 
tions in  Normal  Life,"  Stiid.  Yale  Psychol.  Lab.,  1895,  iii.  1.  Flournoy, 
"  De  1'Influence  de  la  Perception  Visuelle  des  Corps  sur  leur  Poids 
Apparent,"  L'Annee  psychol.,  1894,  i.  198.  Laura  StefFens,  "Uber  die 
motorische  Einstellung,"  Ztsch.  f.  Psychol.  u.  Physiol.  d.  Sinnesorganc, 
1900,  xxiii.  241.  W.  Wundt,  Grundziige  d.  physioL  Psychologic,  Leipzig, 
1902,  5te  Aufl.,  ii.  25.  R.  S.  Woodworth,  Lc  Mouvement,  Paris,  1903. 


CHAPTER  XVII 
ON  LOCAL  SIGNATURE 

Local  Sign. — When  two  sufficiently  distant  points,  either 
of  the  retina  or  of  the  skin,  are  similarly  excited  by  the  same 
stimulus,  the  two  resulting  sensations,  retinal  or  cutaneous, 
differ  in  respect  of  a  character  which  is  termed  "local 
signature."  Through  differences  of  "  local  sign  "  we  become 
aware  of  differences  in  the  direction  of  the  source  of  light, 
or  in  the  point  of  application  of  the  stimulus  to  the  skin. 

The  Cutaneous  Spatial  Threshold. — The  degree,  to  which 
neighbouring  points  on  the  skin  differ  in  local  sign,  varies 
within  wide  limits  over  the  body  surface.  A  measure  of  it 
is  afforded  by  the  "  spatial  threshold,"  the  liminal  distance 
necessary  to  produce  a  double  touch  (exp.  103).  When  two 
points  on  the  skin  are  simultaneously  touched,  they  must 
be  separated  by  the  following  distances  in  order  that  the 
touch  may  be  felt  as  double : — 

Eegion  of  Skin.  Distance  of  the  Points. 

Tip  of  tongue 01  cms. 

Tip  of  finger  (palmar  surface)     .        *  0*2     „ 

Lips  (outer  surface)  v     "•'•]*'•: "•-'-»-       »*  0'5     » 

Tip  of  nose         .         .;     :  „-»"''•      »         •  0*7     „ 

Tip  of  finger  (dorsal  surface)      .       -T~-  0'7     „ 
Lips  (inner  surface)   .         .         .         .2'0,, 

Back  of  hand     .         .         .         .  3*2     „ 

Forearm,  leg,  and  sacrum  .         .         .  4'0     „ 

Sternum 4-5     „ 

Spine ;W:  54     „ 

Arm  and  thigh  ....        ^  6*8     „ 


231 


232  EXPERIMENTAL  PSYCHOLOGY 

Owing  to  individual  variations,  these  data  are  only 
approximate.  They  refer  to  distances  taken  in  a  longi- 
tudinal direction  upon  the  region  mentioned.  When, 
instead,  the  two  points  are  applied  transversely,  the  spatial 
thresholds  are  less  than  those  above  given. 

A  far  lower  threshold  is  obtainable  when  care  is  taken 
to  confine  the  stimulation  to  touch  spots.  Indeed,  it  has 
been  said  that  in  regions  where  touch  spots  are  sufficiently 
scattered  to  permit  of  their  separate  investigation,  the  local 
sign  of  individual  touch  spots  is  so  characteristic,  that,  when 
touched  successively,  each  touch  spot  is  distinguishable  from 
its  neighbour.  Whether  touch  spots  are  selected  or  not, 
the  threshold  is  always  lower,  when  the  two  points  are 
applied  successively,  than  when  they  are  applied  simultane- 
ously, especially  if  the  first  point  be  removed  before  the 
application  of  the  second  point. 

Introspection  near  the  Threshold. — Even  when  the  distance 
between  two  points,  simultaneously  applied  to  the  skin,  is 
not  wide  enough  to  produce  a  double  touch,  the  experience 
may  nevertheless  be  different  from  that  produced  by  the 
application  of  a  single  point.  Although  it  is  an  experience 
of  single  touch,  yet  it  is  altered  in  respect  of  a  character 
which  has  been  called  "extensity."  The  touch  seems  no 
longer  limited  to  a  point.  It  is  now  blurred,  spread  out,  and 
referred  to  a  wider  area.  Ultimately,  as  the  distance  is 
gradually  increased,  this  diffuse  single  touch  gives  way  to 
an  experience  of  double  touch.  We  have  here  an  indication 
of  one  of  the  ways  in  which  the  spatial  threshold  may 
become  lowered  with  practice.  The  individual,  although  he 
does  not  feel  two  distinct  touches,  has,  nevertheless,  an 
experience,  sufficiently  different  from  that  produced  by  the 
application  of  a  single  point,  for  him  to  be  able  to  infer  that 
two  points  are  being  applied  to  his  skin.  This  improved 
power  of  interpretation  may  in  part  account  for  the  abnorm- 
ally low  threshold  which  obtains  among  the  blind. 

When  practice   has   brought   about  a  reduced  spatial 


LOCAL  SIGNATURE  233 

threshold  in  one  cutaneous  area,  a  like  improvement  is 
manifested  in  a  symmetrically  situated  area  on  the  opposite 
unpractised  side  of  the  body.  This  is,  perhaps,  related  to  a 
morbid  condition,  known  as  "  allochiria,"  in  which  the 
patient  is  in  doubt  as  to  which  side  of  his  body  is  touched, 
often  referring  the  touch  to  the  opposite  side.  But  the 
physiological  mechanism,  underlying  this  relation  between 
corresponding  or  symmetrical  areas  of  the  body,  is  at  present 
unknown. 

Variations  of  the  Spatial  Threshold. — The  spatial  thresh- 
old varies  with  the  general  condition  of  the  individual. 
Narcotics  cause  it  to  rise.  It  has  been  employed  unsatis- 
factorily as  a  test  of  mental  fatigue  (page  190). 

Local  conditions  no  doubt  affect  the  spatial  threshold. 
It  is  raised  by  coldness  or  anaemia  of  the  part,  or  by  the 
application  of  irritants,  and  it  is  lowered  by  warmth,  hyper- 
aemia,  or  by  local  friction. 

Loss  of  the  mobility  of  a  limb  (e.g.  through  fracture)  is 
apt  ultimately  to  produce  a  rise  in  the  spatial  threshold. 
Throughout  the  normal  body,  a  similar  correlation  is  observ- 
able between  the  height  of  the  threshold  and  the  exploring 
activity  of  the  region ;  the  spatial  threshold  on  the  limbs, 
for  example,  decreasing  towards  their  free  extremities. 

Cutaneous  Threshold  for  Lines. — We  have  already  seen 
(page  232)  that  two  points,  simultaneously  applied,  may 
give  rise  to  an  experience  of  increased  extensity,  when  their 
distance  apart  is  insufficient  to  produce  an  experience  of 
double  touch.  This  suggests  (and  it  is  experimentally  veri- 
fiable) that  the  shortest  line,  which  can  just  be  appreciated 
when  applied  to  the  skin,  is  less  than  the  distance  necessary 
to  distinguish  clearly  two  simultaneously  applied  points. 
When,  however,  the  subject  is  in  addition  required  to  judge 
the  direction  of  the  line,  the  liminal  length  of  the  line  differs 
little  from  the  liminal  distance  between  two  simultaneously 
applied  points. 

The  apparent  length  of  a  line,  or  of  the  distance  between 


234  EXPERIMENTAL  PSYCHOLOGY 

two  points,  applied  to  the  skin,  depends  on  the  spatial 
threshold  of  the  cutaneous  area  under  investigation. 
According  as  the  differences  in  local  signature  are  coarse 
or  fine,  the  distance  appears  smaller  or  greater  (exp.  104). 

A  similar  result  holds  for  movement.  The  apparent 
rate  of  a  point  moving  along  the  skin  waxes  and  wanes 
with  the  rise  and  fall  of  the  spatial  threshold  (exp.  104). 

The  Histological  Basis  of  the  Spatial  Threshold. — Once 
acquired,  cutaneous  localisation  is  evoked  not  only  when  the 
skin  is  stimulated,  but  also  when  any  part  of  the  afferent 
peripheral,  or  central,  nervous  system,  which  is  functionally 
connected  with  the  skin,  is  stimulated.  Thus  stimulation  of 
the  nerves  within  the  stump  of  an  amputated  limb  usually 
causes  a  localisation  of  the  sensation  in  the  extremities  of 
the  lost  member.  Such  an  illusion  rnay  persist,  though 
somewhat  enfeebled,  throughout  life.  The  amputated  ex- 
tremity, to  which  the  sensations  are  referred,  often  seems  as 
if  it  were  reduced  in  size  and  lay  nearer  than  usual  to  the 
stump.  This  apparent  nearness  is,  perhaps,  the  result  of 
the  loss  of  sensation  from  intermediate  regions.  The 
reduction  in  size  would  then  arise  from  an  incorrect  infer- 
ence of  the  size  of  the  familiar  object,  based  on  its  apparent 
distance. 

It  is  impossible  to  accept  the  view  that  the  cutaneous 
surface  contains  a  number  of  anatomical  areas,  each  supplied 
by  a  different  nerve  fibre,  and  that  a  double  touch  is  ex- 
perienced only  when  the  applied  points  rest  on  different 
anatomical  areas.  In  the  first  place,  there  is  always  great 
overlapping  in  the  nerve  supply  of  any  given  cutaneous 
area.  A  fibre  B  subdivides  to  supply  not  only  the  area  b 
but  also  the  adjoining  areas  a  and  c;  a  fibre  G  similarly 
supplies  the  areas  b  and  d,  as  well  as  c.  In  the  second 
place,  were  the  view  just  mentioned  true,  we  should  expect 
that,  when  the  distance  between  the  two  applied  points  falls 
just  short  of  the  diameter  of  such  an  anatomical  area,  the 
same  equidistant  stimuli  would  give  rise  to  a  double  touch 


LOCAL  SIGNATURE 


235 


reasons  for   supposing  that  a   given 


if  they  lay  on  different  areas,  and  to  a  single  touch  if  they 
lay  on  the  same  area ;  whereas,  in  fact,  nothing  of  this  kind 
occurs. 

There  are  better 
point  of  the  skin, 
when  stimulated, 
evokes  not  only  its 
own  local  sign,  but 
also  to  a  less  ex- 
tent the  local  signs 
of  neighbouring 

points  ;  and  that  the  degree  to  which  the  latter  are  evoked 
decreases  with  their  distance  from  the  point  of  stimulation, 
and  possibly  with  the  delicacy  of  the  local  signature  in  the 
district. 


FIG.  7,  I. 


FIG.  7,  II. 

If  we  represent  the  degree  to  which  two  cutaneous  points 
C  and  Gr  evoke  the  local  signs  of  neighbouring  points  by 
erecting  ordinates  from  a  I  C  d  e  and  e  f  G-  li  i  (fig.  7,  I.), 
joining  the  ends  of  these 
ordinates  to  form  the  curves 
a  p  e,  e  q  i,  clearly  there  is 
no  overlap  in  local  sign,  and 
the  double  touch  is  under 
these  conditions  clearly  re- 
cognised. If,  on  the  other 
hand,  the  points  of  stimulation  be  closer  together,  as 
C'  and  D',  or  G'  and  /'  (fig.  7,  II.),  their  simultaneous 
excitement  must  give  rise  either  to  a  single  less  punctate 
touch,  as  at  p,  or  to  a  broad  blur,  as  at  q  r' ;  the  dotted 


C"  G" 

FIG.  7,  III. 


236  EXPERIMENTAL  PSYCHOLOGY 

lines  representing  the  result  of  compounding  the  two  curves. 
Such  a  condition  cannot  give  rise  to  an  experience  of  clearly 
double  touch ;  whereas,  when  the  peaks  are  sufficiently 
distinct,  as  at  C*  and  G-"  (fig.  7,  III.),  the  threshold  may 
conceivably  have  just  been  reached. 

This  scheme  enables  us  to  understand  the  increase  of 
extensity  which  is  experienced  when  the  two  points,  as  at 
G'  and  /',  are  not  sufficiently  distant  to  allow  of  the  touches 
appearing  double.  Here  points  of  the  skin  lying  between 
G-'  and  I'  have  their  local  signs  evoked  in  almost  equal 
degrees,  as  shown  by  the  dotted  portion  q  r.  The  conditions 
are  almost  identical  with  those  that  would  arise,  were  a  line 
to  press  on  the  skin  between  these  two  points. 

If  we  are  right  in  supposing  that  a  point  of  the  skin 
when  touched  evokes  not  only  its  own  local  sign,  but  also 
the  local  signs  of  neighbouring  points,  we  have  yet  to 
explain  why  this  should  be  the  case.  It  may  be  conceived 
as  the  result  of  association.  The  point  C  has  so  often  been 
touched  simultaneously  with  aide,  and  their  local  signs 
have  been  so  often  evoked  simultaneously,  that  ultimately 
stimulation  of  C  alone  calls  forth  both  its  own  and  the 
neighbouring  local  signs.  Or,  with  more  reason,  the 
phenomenon  may  be  attributed  to  the  gradual  differentiation 
of  local  signature,  which  must  occur  during  the  very  early 
life  of  the  individual.  We  may  suppose  that  at  first  the 
cutaneous  areas,  within  which  no  perceptible  differences  of 
local  sign  exist,  are  comparatively  large,  and  that  as  these 
differences  become  evolved  the  old  sharing  of  local  signature 
nevertheless  persists.  In  other  words,  the  curve,  expressing 
the  degree  to  which  any  point  C  evokes  the  local  sign  of 
neighbouring  cutaneous  points,  is  at  first  broad  and  almost 
peakless,  and  it  gradually  rises  to  acquire  an  apex,  as  shown 
in  fig.  7, 1. 

Relative  and  Absolute  Localisation  on  the  Skin. — So  far 
we  have  dwelt  chiefly  on  the  discrimination  of  two  touched 
points  from  one  another.  We  have  now  to  examine  the 


LOCAL  SIGNATURE  237 

conditions  which  determine  the  localisation  of  the  two 
points  "relatively"  to  one  another,  or  the  localisation  of 
either  of  the  points  " absolutely"  on  the  body  surface, 

In  this  connection,  Aristotle's  experiment  and  its  modi- 
fications (exp.  105)  are  of  interest,  for  they  show  how  much 
our  localisation  of  two  points  on  the  skin  relatively  to  one 
another  depends  on,  and  hence  has  been  derived  from,  the 
normal  position  of  the  body.  That  visual  imagery  need 
play  no  essential  part  in  the  genetic  basis  of  localisation,  is 
shown  by  the  fact  that  the  illusions  of  Aristotle's  experi- 
ment are  obtainable  in  individuals  who  have  been  blind 
from  infancy.  In  normal  people,  however,  visual  imagery 
is  of  undoubted  importance  both  as  regards  the  relative 
localisation  of  two  points  and  as  regards  the  absolute 
localisation  of  a  single  point  on  the  skin.  The  more  help  a 
subject  is  allowed  from  visual  experiences,  the  more  accurate 
becomes  his  "absolute"  localisation  of  any  stimulated 
cutaneous  area  (exp.  106). 

Absolute  localisation  is  also  aided  by  kinsesthetic 
(motor)  sensations.  If  one  of  the  subject's  fingers  be 
touched  when  his  eyes  are  closed,  he  is  sometimes  apt  to 
localise  the  stimulus  on  the  correct  part  of  the  finger,  but 
on  another  finger.  The  error  is  often  avoidable  if  he  be 
allowed  to  move  one  or  other  of  his  fingers,  not  necessarily 
that  which  is  touched.  The  value  of  kineesthetic  sensations 
in  localisation  appears  to  be  evident  in  the  following  patho- 
logical case.  In  one  limb,  the  cutaneous  sensibility  in 
which  had  become  slightly  and  the  kina^sthesis  enormously 
reduced,  the  error  of  localisation  was  far  greater  than  on 
the  limb  of  the  opposite  side,  in  which  the  sense  of  move- 
ment was  preserved  but  cutaneous  sensibility  was  very 
much  reduced. 

Thus  absolute  localisation  clearly  involves  many  other 
factors  than  that  of  local  signature.  Otherwise,  indeed,  we 
might  expect  that,  if  a  point  B  were  located  at  A,  the  point 
A  when  touched  would,  by  reciprocal  confusion,  be  located 


238  EXPERIMENTAL  PSYCHOLOGY 

at  B.  On  the  contrary,  the  error  of  localisation  has 
generally  a  constant  direction.  Further,  the  degree  of  error 
is  found  not  to  be  proportionate  to  the  spatial  threshold. 

The  Basis  of  Cutaneous  and  Retinal  Local  Signature. — We 
have  left  on  one  side  the  problem,  how  cutaneous  and 
retinal  points  acquire  their  local  sign. 

Lotze,  to  whom  we  owe  the  term  "  local  sign,"  supposed 
that  the  differences  in  cutaneous  local  sign  are  the  result  of 
those  differences  in  structure  and  in  nerve  supply  of  the 
skin  surfaces,  by  virtue  of  which  the  direct  and  associated 
sensations,  produced  by  stimulation  of  one  point,  differ  from 
those  produced  by  stimulation  of  another,  6.  Others  have 
attributed  local  signature  solely  to  the  various  impulses  to 
movement,  or  to  the  kinaesthetic  sensations  derived  from 
movement,  which  a  touched  spot  of  the  skin  calls  forth. 

Hering  supposes  that  retinal  local  signs  are  innate,  each 
point  of  the  retina  differing  from  other  points  in  height-  or 
breadth-value,  or  in  both  values.  A  detailed  examination  of 
Bering's  views  is  impossible  here.  But  stress  may  be  laid 
on  his  supposition  that  these  congenital  differences  of  local 
sign  are  of  purely  retinal  origin,  and  that  they  reflexly 
produce  the  movements  of  the  orbits  leading  to  direct 
fixation. 

Others  have  supposed  that  retinal  local  signs  are  due  to 
the  kinsesthetic  sensations  arising  from  orbital  movement. 
It  has  been  argued  that,  when  a  ray  of  light  falls  upon  a 
retinal  spot,  a  movement  of  the  eye,  accompanied  by  de- 
finite kinsesthetic  sensations,  occurs  in  order  that  the  image 
may  be  brought  to  fall  on  the  fovea.  According  to  this 
view,  the  kinsesthetic  sensations,  varying  according  to  the 
distance  and  direction  of  the  necessary  orbital  movement, 
become  associated  with  the  retinal  spots  which,  when 
stimulated,  produce  them ;  so  that,  finally,  although  no  eye 
movements  may  be  made,  stimulation  of  various  retinal 
spots  yet  evokes  their  characteristic  local  signs.  Here  we 
approach  Lotze's  standpoint,  who  regarded  retinal  local 


LOCAL  SIGNATURE  239 

signs  as  the  result  of  centrally  aroused  impulses  to  move- 
ment,— a  standpoint  fundamentally  different  from  Bering's, 
who,  as  we  have  just  seen,  regarded  retinal  local  signs  as  the 
cause  of  movement. 

Against  the  kinsesthetic  basis  of  retinal  local  signature 
several  weighty  objections  may  be  urged.  In  cases  of 
recovery  from  congenital  blindness,  the  individual  is  at  once 
able  to  recognise  differences  in  size  and  in  form  of  images 
received  by  his  retina.  So  instantly  is  a  round  object  seen 
to  differ  from  a  triangular  one,  that  the  judgment  is  clearly 
independent  of  the  training  which  would  be  needed  before 
the  eyes  could  be  moved  in  a  fashion  orderly  enough  to 
follow  the  outlines  of  the  object. 

Again,  in  certain  cases  of  retinitis,  where  the  elements  in 
the  affected  region  of  the  retina  are  abnormally  crowded 
together  or  separated  by  inflammatory  products,  a  local 
"  metamorphopsia,"  or  distortion  of  vision,  is  produced.  It 
arises  from  the  fact  that  the  displaced  retinal  elements 
retain  their  normal  local  sign  under  these  conditions.  A 
line,  whose  image  falls  on  the  affected  area  of  the  retina, 
appears  bent  in  or  bowed  out,  according  as  the  elements  are 
unduly  separated  or  contracted.  Such  distortion  would 
surely  be  corrected  in  time  if  motor  sensations  were  the 
basis  of  local  signature.  In  point  of  fact,  it  is  said  to 
persist  for  months  or  even  for  years, — indeed,  as  long, 
perhaps,  as  the  pathological  condition  remains. 

Lastly,  attention  may  be  called  to  the  fact  that  lines 
which  subtend  less  than  an  angle  of  one  minute  can  be 
spatially  distinguished  by  the  eye.  If  such  discrimination 
depended  for  its  origin  on  kinsesthetie  sensibility,  then  the 
delicacy  of  kinsesthesis  in  the  orbits  must  be  almost  in- 
credibly greater  than  that  in  any  other  part  of  the  body. 

We  can  hardly  avoid  the  conclusion  that  the  local 
signature  of  the  retina  is  innate,  and  that  the  basis  of  it  is 
to  be  sought  rather  on  the  sensory  than  on  the  motor  side  of 
the  retinal  sensori-motor  mechanism.  A  similar  conclusion 


240  EXPERIMENTAL  PSYCHOLOGY 

is  probably  true  as  regards  the  skin.  Yet  even  so,  we  must 
admit  that,  in  conjunction  with  these  innate  differences  of 
local  signature,  kinsesthetic  sensations  play  an  important  role, 
during  infancy,  in  the  elaboration  and  orderly  arrangement  of 
spatial  relations.  To  realise  the  importance  of  the  role,  one 
has  only  to  attempt  to  conceive  what  sort  of  space  a  subject 
would  create  for  himself  were  he  endowed  with  visual  and 
cutaneous  sensibility,  but  congenitally  deprived  of  kimesthesis. 

Moving  Retinal  Images. — When  the  image  of  a  moving 
object  falls  on  the  stationary  retina,  it  is  found  that  the 
smallest  rate  of  movement  which  can  be  detected  is  de- 
pendent upon  the  simultaneous  presence  of  stationary 
objects.  An  angular  movement  of  between  V  and  2'  per 
second  can  be  immediately  detected  when  the  marks  on  a 
rotating  drum  are  regarded.  This  threshold,  however,  is 
immediately  raised  to  about  20'  per  second  when  the  drum 
is  viewed  through  a  slit  so  that  surrounding  resting  objects 
are  cut  off  from  view.  But  there  is  evidence  that  even 
under  these  conditions  the  stationary  margins  of  the  slit 
serve  as  a  guide.  For  when  a  glowing  thread  is  alternately 
slowly  moved  and  brought  to  rest  in  a  dark  room,  the  move- 
ment can  be  detected  only  with  difficulty,  and  illusions  of 
movement  frequently  occur.  The  latter  have  been  attributed 
to  involuntary  and  unconscious  movements  of  the  eyes,  but 
this  has  been  denied  by  other  observers,  who  have  termed 
them  "  autokinetic  "  sensations.  A  stationary  point  of  light, 
regarded  in  a  dark  room,  may  appear  to  move  with  an 
angular  velocity  of  from  2°  to  3°  per  second. 

The  threshold  for  movement  is  higher  in  peripheral  than 
in  foveal  vision.  We  are  ignorant  of  any  influence  of  dark 
adaptation  upon  the  threshold.  Moving  objects  unquestion- 
ably travel  with  apparently  greater  speed  in  peripheral  than 
in  foveal  vision. 

When  part  of  the  visual  field  is  moving,  it  tends  simul- 
taneously to  induce  an  apparent  contrary  movement  in 
surrounding  stationary  parts  of  the  field  (exp.  108). 


LOCAL  SIGNATURE  241 

Retinal  After -sensations  of  Movement. — A  transient  "  posi- 
tive "  after-effect  has  been  described  but  disputed.  It  has 
been  said  to  occur  when  the  eyes  are  closed  after  having 
followed  a  moving  object,  or  after  having  at  rest  received 
the  image  of  a  moving  object. 

The  "negative  "  effects,  which  are  easier  to  observe,  are  not 
open  to  dispute.  Eesting  objects,  when  regarded  after  the 
eyes  have  been  watching  moving  objects,  appear  to  be 
moving  in  a  direction  contrary  to  the  previous  movement 
(exp.  107,  108).  This  after-effect  was  attributed  by  Helm- 
holtz  to  unconsciously  continued  movements  of  the  eyes; 
the  subject  believing  that  his  eyes  are  stationary,  and  so 
inferring  that  the  field  is  moving  in  the  reverse  direction. 

Such  an  explanation,  however,  fails  to  account  for  the 
following  facts.  If  a  black  spiral  band  painted  on  a  white 
disc  be  slowly  rotated  and  be  fixated  during  rotation,  the 
disc,  on  being  brought  to  rest,  will  show  most  striking  changes 
which  no  after-movements  of  the  eyes  can  explain.  It  will 
appear  to  contract  or  to  expand  with  a  peculiar  streaming 
movement,  according  to  the  direction  of  rotation.  If  only 
part  of  the  retina  receive  the  image  of  the  moving  field, 
the  after-movement  is  confined  to  the  part  stimulated  (exp. 
109). 

If  a  disc  be  prepared  with  two  different  superposed 
spirals,  or  with  a  spiral  which  changes  its  direction  several 
times  as  it  proceeds  from  the  centre  to  the  periphery  of  the 
disc,  the  various  directions  of  movement  are  simultaneously 
represented  in  the  after-effect.  When  two  contrary  move- 
ments are  presented,  one  to  one  eye,  the  other  to  the  other, 
the  af ter-effects  may  neutralise  one  another ;  or  a  single 
after-movement,  the  resultant  of  the  two  components,  may 
be  seen ;  or  now  one  after-movement,  now  the  other,  may 
occur  alternately. 

Such  results  appear  to  show  a  binocular  relation  not 
unlike  that  which,  as  we  shall  see,  obtains  in  binocular  colour 
combination  and  contrast  (pages  280,  281).     They  would  lead 
16 


242  EXPERIMENTAL  PSYCHOLOGY 

us  to  ascribe  a  sensory  character  to  our  visual  (and  perhaps 
also  to  our  cutaneous)  experiences  of  movement,  and  generally 
to  compare  negative  after-movements  to  negative  after- 
images, ascribing  a  like  cerebro-retinal  basis  to  each. 
Certainly  from  the  phylogenetic  standpoint  the  visual  per- 
ception of  movement  must  be  as  primitive  as,  and  perhaps 
more  important  than,  the  appearance  of  well-defined  visual 
images. 

BIBLIOGRAPHY. 

E.  H.  Weber,  "Der  Tastsinn  u.  d.  Gemeingefuhl "  in  Wagner's 
Handwdrterbucli  d.  Physiol.,  Braunschweig,  1846,  iii.  (2)  481.  R.  H. 
Lotze,  Medicinische  Psychologie,  Leipzig,  1852,  325.  E.  Hering,  Bdtrcige 
zur  Physiol.,  Leipzig,  1864,  Heft  v.  V.  Henri,  Ueber  d.  Raumivahrnehm- 
ungen  d.  Tastsinnes,  Berlin,  1898.  A.  Binet,  "La  Mesure  de  la 
Sensibilite,"  and  other  articles,  L'Anne'e  psychologique,  1903,  9me  Annee, 
89.  W.  McDougall,  Reports  of  the  Cambridge  Anthropological  Expedition 
to  the  Torres  Straits,  1903,  ii.  189.  C.  Spearman,  "  Analysis  of  Localisation, 
illustrated  by  a  Brown -Sequard  Case,"  Brit.  Journ.  of  Psijchol.,  1905,  i.  286  ; 
"  Fortschritte  auf  d.  Gebiete  d.  Psychophysik  d.  raumlichen  Vorstellungen," 
Arch.  f.  d.  ges.  Psychol.,  1906,  viii.  (Literaturbericht).  E.  Jones,  "The 
Precise  Diagnostic  Value  of  Allochiria, "  Brain,  1907,  xxx.  490.  A.  v.  Szily, 
"  Bewegungsnachbild  u.  Bewegungscontrast,"  Ztsch.  f.  Psychol.  u.  Physiol. 
d.  Sinncsorgane,  1905,  xxxviii.  81;  "ZumStudiumd.  Bewegungsnachbildes," 
Ztsch.  f.  Sinncsphysiol.,  1907,  xlii.  109.  A.  Easier,  "  Ueber  d.  Sehen  v. 
Bewegungen,"  Arch.f.  d.  ges.  Physiol.,  1906,  cxv.  582  ;  ibid.,  1908,  cxxiv. 
313,  L.  Ruppert,  "  Ein  Vergleich  zur  d.  Distinktionsvermogen  u.  d. 
Bewegungsempfindlichkeit  d.  Netzhautperipherie,"  Ztsch.  f.  Sinnesphysiol. 
1908,  xlii.  409. 


CHAPTER   XVIII 
ON  SENSIBILITY  AND  SENSORY  ACUITY  l 

Discrimination  as  a  Factor  in  Sensation. — The  sensations 
of  the  so-called  intrinsic  light  in  the  retina,  the  auditory 
sensations  that  occur  in  absolute  silence,  the  temperature 
sensations  that  accompany  blushing  or  pallor,  are  all  illustra- 
tions of  the  fact  that,  however  carefully  the  sense  organs  be 
guarded  from  external  stimuli,  they  cannot  be  considered  to 
be  absolutely  at  rest.  The  effect  of  applying  an  external 
stimulus  to  an  apparently  resting  sense  organ  is  rather  to 
alter  the  contribution  made  by  that  organ  to  the  sum  total 
of  experience  than  to  transform  it  from  a  state  of  absolute 
quiescence  into  one  of  activity.  In  determining,  for 
example,  the  acuity  or  sensibility  of  hearing,  the  attitude 
of  the  subject  is  found  to  involve  discrimination  between 
his  auditory  experiences  before  and  those  at  the  moment 
of,  the  presentation  of  the  stimulus.  In  general,  a  test  for 
sensory  acuity  or  sensibility  entails  sensory  discrimination. 

The  Complexity  of  the  Conditions  determining  Sensibility. — 
Sensibility  is  dependent  not  merely  on  the  condition  of  the 
sensory  structure  which  is  under  examination,  but  also 
on  the  condition  of  neighbouring  sensory  structures,  and 
indeed  of  all  other  parts  of  the  nervous  system.  The 
sensibility  of  a  given  retinal  area,  for  example,  varies  with 
the  state  of  adaptation  of  that  area  and  of  the  surrounding 
areas  of  the  retina.  It  depends  also  on  the  presence,  and 
on  the  nature  and  intensity,  of  sensations  which  are 

1  See  footnote  to  Chapter  III. 
243 


244  EXPERIMENTAL  PSYCHOLOGY 

simultaneously  or  have  recently  been  evoked  from  other 
end  organs.  It  is  affected  by  general  or  by  local  fatigue, 
by  attention,  practice,  expectation,  and  interpretation. 
Sensibility,  in  fact,  varies  with  the  total  mental  state  at 
the  moment  of  determination. 

[On  physiological  and  anatomical  grounds  we  may  be 
disposed  to  regard  sensation  as  the  simple  reaction  of  the 
sensory  apparatus.  We  may  attempt  to  distinguish 
between  the  sensibility  of  the  sensory  organ, — dependent 
on  simpler,  more  peripheral  conditions, — on  the  one  hand, 
and  the  effect  of  that  sensibility  on  consciousness, — the 
resultant  of  far  more  complex  factors, — on  the  other.  From 
this  standpoint,  sensibility  may  conceivably  remain  un- 
changed when  a  stimulus,  previously  effective,  is  now 
ineffective.  Such  alteration  in  the  efficiency  of  the 
stimulus  may  be  considered  due  merely  to  changes  in  the 
higher  parts  of  the  brain. 

But,  from  the  psychological  standpoint,  we  have  to 
remember  that  the  occurrence  of  a  sensation  implies  a  change 
in  the  complex  totality  of  consciousness,  a  change  modified 
by  past  and  present  experiences,  and  modifying  present  and 
future  experiences.  Consequently,  if  from  one  aspect  we 
are  led  to  regard  sensations  as  a  number  of  elementary 
independent  entities,  from  the  other  we  are  led  to  regard 
them  as  modifications  of  the  Ego.  Microcosms  there  may 
be  within  the  macrocosm  of  the  Ego,  but  the  normal  Ego  has 
no  knowledge  of  any  consciousness  save  its  own.  From  the 
psychological  standpoint,  the  activity  of  the  parts  is  in- 
separably bound  up  in  the  activity  of  the  whole.] 

Visual  Acuity.  —  Visual  acuity  is  measured  by  the 
angle  subtended  at  the  eye  by  two  points  which  can  be 
just  distinguished.  It  is  commonly  determined  by  means 
of  "  test-types,"  composed  of  letters  of  different  size,  which 
are  numbered  according  to  the  distance  in  metres  at  which 
they  should  be  correctly  read  by  persons  possessing  "  normal " 
visual  acuity.  At  this  distance  the  height  of  a  letter 


SENSIBILITY  AND  SENSORY  ACUITY      245 

subtends  an  angle  of  five  minutes,  and  the  smaller  squares 
within  the  outline  of  a  letter  subtend  an  angle  of  one 
minute  at  the  eye.  An  angle  of  one  minute  is  approximately 
that  which  is  required  for  the  clear  discrimination  of  fine 
black  lines,  separated  by  a  distance  of  their  own  diameter 
from  one  another. 

Conducted  in  this  way,  the  determination  of  visual 
acuity  involves  many  complex  factors.  Variations  in 
brightness,  contrast,  and  irradiation  are  the  most  obvious 
of  these.  The  general  illumination  has  an  important 
influence.  The  acuity  determined  in  a  room  is  very 
different  from  that  determined  out  of  doors. 

Both  indoors  and  especially- in  the  open,  many  people 
have  a  visual  acuity  exceeding  what  is  commonly  accepted 
as  the  normal.  This  is  partly  due  to  the  fact  that  the 
letters  may  be  read  even  when  their  form  is  not  perfectly 
seen.  The  individual  is  able,  after  a  little  practice,  correctly 
to  distinguish  similar  letters  from  one  another,  not  because 
he  has  a  clear  image  of  their  respective  outlines,  but  because 
he  is  able  to  draw  inferences.  He  notes,  for  example,  that 
the  side  of  a  P  contains  a  dim  but  continuous  blur,  while  in 
an  F  there  is  a  clearly  open  space.  It  is  evident,  then,  that 
the  distance  at  which  letters  of  a  given  size  can  be  dis- 
tinguished depends  upon  the  particular  letters  used  in  the 
test,  and  upon  the  degree  of  the  subject's  familiarity  with 
them. 

To  obviate  the  individual  differences  thus  resulting  from 
illiteracy,  test  types  composed  of  a  single  form  or  letter,  e.g. 
C »  E  or  C>  have  been  constructed,  the  form  or  letter 
varying  only  in  size  and  position  (exp.  111).  But  even 
with  such  single  forms,  interpretation  and  inferences  are 
still  possible.  The  subject  is  able  to  guess  the  position  of 
the  letters  correctly  when  he  receives  only  a  very  blurred 
image  of  their  form. 

Different  arrangements  of  black  (or  white)  dots  on  a 
white  (or  black)  background, — various  dots  being  exposed 


246  EXPERIMENTAL  PSYCHOLOGY 

and  counted  at  different  distances, — have  been  also  employed 
for  the  estimation  of  visual  acuity.  But  here  again  different 
results  may  be  reached  by  two  persons  who,  from  the  strictly 
"  physiological "  standpoint,  have  equal  visual  acuity.  The  one, 
an  illiterate,  has  greater  difficulty  of  counting  than  the  other,  a 
more  cultured  individual ;  or  the  one  makes  no  attempt,  while 
the  other  (owing  to  greater  interest  and  perhaps  to  greater 
intelligence)  leaves  no  effort  unspared,  to  interpret  as  dots 
what  he  but  dimly  sees ;  the  blunt  stolid  character  of  the 
one  leading  him  to  describe  nothing  but  what  is  obvious 
and  certain,  the  highly  imaginative  character  of  the  other 
inducing  him  to  go  further  by  discrimination  and  inter- 
pretation. 

Visual  Efficiency. — It  is  important  to  distinguish  between 
visual  acuity  and  what  has  been  termed  "  visual  efficiency." 
The  latter  makes  no  allowance  for  errors  due  to  refraction 
or  to  other  ocular  defects  in  front  of  the  retina,  whereas  in 
the  determination  of  visual  acuity  these  disturbances  are 
presumed  to  be  absent,  or  to  have  been  eliminated  by  the 
use  of  glasses. 

Visual  Sensibility. — Visual  acuity,  as  already  denned, 
must  be  further  distinguished  from  "visual  sensibility." 
This  is  measured  by  the  lowest  intensity  of  light  which 
can  produce  a  sensation,  when  it  falls  on  a  given  area  of 
the  retina.  Visual  sensibility  increases  with  the  extent 
of  the  area  stimulated,  especially  in  the  periphery  of  the 
retina.  It  increases  enormously  (in  some  cases  even  eight 
thousandfold)  with  increased  dark  adaptation  of  the  eye, 
if  the  area  stimulated  lie  outside  the  fovea.  While  in  the 
bright  adapted  eye  the  fovea  is  most  sensitive,  in  the  dark 
adapted  eye  the  extra-foveal  area  of  the  retina  is  most 
sensitive.  Indeed,  astronomers  have  for  long  been  aware  that 
a  dim  star  can  be  best  seen  not  by  direct  vision,  but  by 
fixation  of  a  point  which  lies  to  one  side  of  the  star.  The 
temporal  half  of  the  bright-adapted  retina  has  been  found 
to  be  less  sensitive  to  light  than  the  nasal  half. 


SENSIBILITY  AND  SENSORY  ACUITY       247 

The  comparative  sensibility  of  the  eye  for  different 
colours  has  been  estimated  by  observing  the  point  at  which 
the  various  colour  sensations  disappear,  as  spectral  hues  are 
gradually  reduced  in  intensity.  Eed  disappears  first,  then 
yellow  and  blue,  next  violet,  while  green  persists  longest. 
The  results,  however,  are  dependent  on  the  saturation  of 
the  colour,  on  the  size  of  the  field,  and  on  the  local  and 
general  condition  of  the  retina. 

Auditory  acuity. — The  estimation  of  auditory  acuity — 
or  sensibility — (for  in  all  sense  organs  other  than  the  eye 
the  two  terms  are  synonymous)  is  complicated  by  many  of 
the  factors  which,  as  we  have  seen,  influence  the  determi- 
nation of  visual  acuity.  Persons  whose  hearing  is  obtuse 
differ  widely  in  their  sensitivity  to  different  kinds  of  auditory 
stimuli,  e.g.  to  words,  tones,  and  noises.  This  is,  in  great 
part,  due  to  individual  differences  in  interpretation. 

The  human  voice  is  obviously  a  very  uncertain  source  of 
stimulus.  It  varies  as  regards  distinctness,  loudness,  and 
quality  not  only  in  different  individuals,  but  also  in  the 
same  individual,  however  careful  he  be,  at  different  times. 
Clearly,  too,  some  words  are  audible  at  a  greater  distance 
than  others,  according  to  the  nature,  arrangement,  and 
relative  number  of  vowels  and  consonants  contained  therein. 

Whatever  be  the  form  of  stimulus,  its  constancy  and 
uniformity  are  of  great  importance.  For  this  reason,  the 
sound  resulting  from  the  fall  of  an  object  from  a  given 
height  appears,  at  first  sight,  to  be  especially  suitable.  The 
intensity  of  the  stimulus  can  be  varied  either  by  altering 
the  height  from  which  the  body  is  let  fall,  or  by  increasing 
or  decreasing  the  distance  between  the  instrument  and  the 
individual's  ear.  The  disadvantages  of  such  a  method  of 
estimation  arise  partly  from  alterations  in  the  pitch  or 
quality  of  the  sound  according  to  the  acceleration  of  the 
falling  body  at  the  moment  of  impact,  and  partly  from  the 
complex  nature  of  the  sound,  which  consequently  appears 
to  vary  in  quality  according  to  its  distance  from  the  ear 


248  EXPERIMENTAL  PSYCHOLOGY 

(page  291).  Fairly  satisfactory  results  can  be  obtained  by 
the  use  of  Politzer's  acoumeter  or  of  a  stop-watch  (exp.  112). 

Auditory  acuity  has  been  also  tested  by  means  of  a 
stationary  vibrating  tuning  fork,  which  is  allowed  to  "  ring 
off"  until  the  subject  no  longer  hears  it,  or  by  means  of  a 
watch,  which  is  gradually  removed  from  the  ear  until  the 
ticks  are  inaudible.  But  estimations  based  on  such  methods 
are  in  various  degrees  unsatisfactory. 

A  very  great  difficulty  arises  through  the  variable  play 
of  certain  internal  and  external  factors,  the  acuity  changing 
from  hour  to  hour,  or  from  day  to  day,  according  to  the 
condition  of  the  individual,  and  according  to  the  disturbing 
influence  of  adventitious  sounds. 

At  or  near  the  threshold  the  judgments  of  the  subject 
are  complicated  by  the  influence  of  expectation  and  of 
auditory  memory  images.  He  attributes  to  subjective 
experiences  those  which  really  are  of  objective  origin,  and 
declares  that  he  hears  the  sound  during  periods  of  actual 
silence. 

At  present  we  know  little  of  the  relation  between 
auditory  acuity  and  differences  of  pitch.  We  are  aware 
that  both  the  low  and  the  high  limits  of  the  tone  range 
are  dependent  on  the  intensity  of  the  tone  stimuli  (page 
36),  and  there  is  reason  to  think  that,  when  two  tones  of 
equal  physical  intensity  are  successively  sounded,  the  higher 
tone  gives  rise  to  the  more  intense  sensation. 

Olfactory  acuity. — Olfactory  acuity  may  be  measured 
either  by  Zwaardemaker's  olfactometer  (fig.  8),  or  by  glass 
vessels  some  of  which  contain  water,  others  the  odorous 
solution  in  different  strengths  (exp.  113). 

Zwaardemaker,  employing  a  rubber  tube  as  the  standard 
substance  for  his  olfactometer,  found  that  individuals  who 
had  average  olfactory  acuity  could  just  detect  its  odour 
when  it  was  drawn  out  to  a  distance  of  seven  millimetres 
beyond  the  inner  glass  tube.  This  length  he  took  as  his 
unit,  calling  it  an  "olfactie."  An  individual  who  fails  to 


SENSIBILITY  AND  SENSORY  ACUITY       249 


detect  the  odour  until  twenty -one  millimetres  of  the  rubber 
tube  had  been  exposed  is  said  to  have  an  acuity  of  three 
olfacties.  Tubes  of  various  solids  (e.g.  beeswax,  paraffin) 
have  been  employed.  Odorous  liquids  may  be  also  used, 
a  porous  tube  being  soaked  in  a  solution  of  the  substance 
before  it  is  placed  over  the  inner  tube  of  the  instrument. 
Care  must  always  be  taken  to  cleanse  the  inner  tube 
repeatedly. 

Olfactory  acuity,  as  thus  determined,  varies  in  different 
individuals  according  to  the 
odour  which  is  employed.  Those 
who  have  unusually  keen  sensi- 
bility for  one  odour  may  have 
only  normal  or  even  subnormal 
sensibility  for  others. 

A  difficulty  often  arises  in 
the  estimation  of  the  threshold 
owing  to  a  change  in  the 
character  of  the  odour,  when 
the  vapour  is  in  a  very  diluted 
state.  Under  these  circum- 
stances the  subject  may  assert 
that  he  can  smell  something, 
but  that  it  is  not  at  all  the 
ordinary  odour  of  the  substance 
which  he  is  expected  to  smell. 

Here  again  individual  differences  in  interpretation  and 
inference  have  ample  play. 

Gustatory  Acuity. — Gustatory  acuity  may  be  tested  by 
the  application  of  solutions  of  various  strengths  either  to 
individual  papilla  or  to  larger  areas  of  the  tongue  or  other 
neighbouring,  similarly  sensitive,  regions.  The  principal 
factors,  which  are  liable  to  disturb  the  estimation,  have 
been  already  indicated. 

Tactual  Acuity. — Tactual  acuity,  so  far  as  it  concerns  the 
sensibility  of  the  skin  to  light  touch,  may  be  estimated  by 


FIG.  8. — Zwaardemaker's  Olfacto- 
mcter.  This  instrument  con- 
sists of  a  cylindrical  tube,  C, 
containing  the  odorous  substance 
and  fitting  over  the  graduated 
glass  tube,  T,  which  passes 
through  the  wooden  screen,  S, 
to  be  inserted  into  the  nostril 
of  the  observer. 


250  EXPERIMENTAL  PSYCHOLOGY 

means  of  hairs  or  glass  fibres  which  have  been  previously 
standardised,  or  by  carefully  applying  plates  of  varying 
sizes  and  weight,  previously  warmed  to  the  skin  temper- 
ature. It  is  found  that  the  threshold  varies  according  to 
the  region,  the  extent,  and  the  condition  of  the  cutaneous 
area  which  is  stimulated,  and  according  to  the  rate  of 
application  of  the  stimulus.  Up  to  a  certain  limit  a  small 
pressure  applied  to  a  larger  area  will  be  effective  when  the 
same  pressure  applied  to  a  smaller  area  proves  subliminal. 
Increased  tension  or  previous  friction  of  the  skin  raises  the 
threshold  of  that  area. 

The  closeness  of  aggregation  and  the  variations  in  sen- 
sitivity of  the  touch  spots,  and  the  thickness  of  the  skin 
differ  in  different  regions.  Movement  of  the  hairs  of  the 
skin  is  from  three  to  twelve  times  more  effective  than 
cutaneous  pressure  in  producing  a  sensation  of  touch. 

The  following  pressures  per  square  millimetre  express 
the  touch  thresholds  for  different  regions  of  the  body,  as 
determined  by  stimulation  with  glass  wool  fibres,  the  surface 
of  contact  varying  from  -5-^  to  TV  sq.  mm. 

2  grams — Tongue  and  nose. 
2*5      „          Lips. 

3  „          Finger  tip  and  forehead. 
5         „          Dorsal  surface  of  finger. 

7  „          Palm,  arm,  and  thigh. 

8  „          Forearm. 
12         „          Occiput. 

16         „          Calf  and  shoulder. 

26  „  Extensor  surface  of  arm,  abdomen,  ab- 
ductor surface  of  thigh. 

28         „          Shin  and  sole. 

33         „          Extensor  surface  of  forearm. 

48         „          Loins. 

Thermal  Acuity.  —  Thermal  acuity  may  be  similarly 
estimated  by  punctate  exploration  or  by  the  application  of 
properly  warmed  or  cooled  surfaces  to  larger  areas  of  the 


SENSIBILITY  AND  SENSORY  ACUITY      251 

skin.  It  is  impossible  to  determine  the  threshold  of 
thermal  sensibility  without  taking  into  account  the  tem- 
perature of  adaptation  of  the  area  under  examination 
(page  16).  The  sensitivity  varies  according  to  the  dura- 
tion of  application  of  the  stimulus,  the  region,  extent,  and 
condition  of  the  cutaneous  area. 

The  clothed  parts  of  the  body,  e.g.  the  nipples  and  the 
loins,  are  generally  more  sensitive  to  cold  and  heat  than 
the  unclothed  parts.  The  lower  eyelids,  cheeks,  and 
temples  are  marked  exceptions  to  this  rule.  The  middle 
line  of  the  body  is  relatively  insensitive  to  temperature. 
The  sensitivity  increases  from  the  far  extremity  of  the 
limbs  towards  the  trunk. 

Algesic  Acuity. — The  sensibility  to  pain  may  be  simil- 
arly tested  by  punctate  or  by  surface  exploration,  the 
stimulus  being  generally  a  prick  or  a  severe  pressure.  The 
threshold  for  pain  is  higher  than  that  for  touch,  but  the 
ratio  of  the  two  thresholds  varies  enormously  in  different 
regions  of  the  body.  The  threshold  is  highest  on  the 
buttock,  thigh,  penis,  ankle,  and  palm,  and  is  lowest  on  the 
temple,  dorsal  surface  of  the  fingers,  and  tip  of  the  tongue. 
It  is  higher  on  the  flexor  than  on  the  extensor  surface  of 
the  limbs,  and  is  highest  where  the  skin  is  thick,  or  where 
it  overlies  thick  muscle,  and  does  not  overlie  bone.  The 
threshold  is  higher  among  primitive  than  among  civilised 
people. 

Kincesthetic  Acuity. — The  threshold  of  kinsesthesis  may 
be  determined  by  observing  the  smallest  appreciable  amount 
of  movement,  when  the  eyes  are  closed  and  all  adventitious 
influences  are  eliminated  (exp.  44).  But  the  liminal  move- 
ment depends  so  much  on  the  rate  at  which  that  movement 
takes  place  that  the  latter  must  be  studied  conjointly  with 
the  former. 

If  account  be  taken  of  both  these  factors,  the  kin- 
aesthetic  sensibility  of  different  parts  of  the  body  may  be 
compared.  It  appears  that  our  appreciation  of  movement 


252  EXPERIMENTAL  PSYCHOLOGY 

in  the  proximal  joints  (the  shoulder  or  hip)  is  keener  than 
in  the  more  distal  ones  (the  finger  or  ankle). 

Sensory  Adaptation  and  Fatigue. — We  have  already 
(page  243)  called  attention  to  the  important  influence  of 
sensory  adaptation  and  sensory  fatigue  upon  sensibility. 
The  broad  difference  in  nature  between  these  two  factors 
is  sufficiently  obvious,  but  the  differentiation  of  their  effects 
from  one  another  is  often  extremely  difficult.  Sometimes 
the  one,  sometimes  the  other,  is  the  preponderating  factor, 
while  in  the  case  of  some  senses  it  is  at  present  impossible 
to  determine  whether  the  condition  be  one  of  adaptation  or 
fatigue. 

In  the  retina,  sensory  adaptation  is  so  prominent  (pages 
79,  96),  that  the  evidence  in  favour  of  fatigue  is  extremely 
slight.  The  eye  endures  the  continuous  light  of  a  northern 
summer  without  signs  of  exhaustion.  The  effects  of  wear- 
ing coloured  glasses  (exp.  114)  are  clearly  ascribable  to 
adaptation  rather  than  to  fatigue. 

In  the  case  of  the  skin,  on  the  other  hand,  the  hot  and 
cold  spots  are  extremely  fatigable.  They  appear  to  react 
explosively  and  then  to  require  time  for  the  regeneration  of 
their  functional  powers.  But,  in  addition  to  this  liability 
of  the  spots  to  fatigue,  we  have  ample  evidence  of  adapta- 
tion in  the  case  of  cutaneous  sensations  of  temperature 
(page  16).  Sensory  adaptation  is  also  of  great  importance 
in  tactual  sensibility,  and  probably  occurs  in  a  certain 
degree  in  sensations  of  pain. 

Tones  which  are  near  the  upper  limit  of  hearing  give 
rise  to  intermittent  sensations.  It  is,  however,  more  prob- 
able that  the  interruptions  are  due  to  external  physical 
conditions  rather  than  to  sensory  fatigue.  Other  evidence, 
brought  forward  in  favour  of  auditory  fatigue,  may  be  as 
well  adduced  in  favour  of  sensory  adaptation  (exps.  117, 
118). 

The  play  of  sensations,  brought  about  by  the  prolonged 
application  of  an  odorous  stimulus  (page  115  ;  exp.  83), 


SENSIBILITY  AND  SENSORY  ACUITY      253 

may  be  most  readily  ascribed  to  fatigue,  but  the  possible 
influence  of  adaptation  cannot  be  overlooked. 

[Other  Forms  of  Adaptation. — In  the  case  of  labyrinthine 
sensations,  the  form  of  adaptation  which  occurs  is  some- 
what different  from  the  strictly  sensory  form.  During 
rest  after  a  prolonged  sea-voyage  or  railway  journey,  the 
after-effects  of  such  adaptation  are  too  well  known  to  need 
description. 

Yet  another  form  of  adaptation  occurs,  as  we  have  seen 
in  muscular  activity.  The  system  becomes  attuned  to  pro- 
ducing a  particular  degree  of  muscular  contraction  (page 
222)  or  a  particular  sequence  of  contractions  (page  221) ; 
and  consequently,  when  a  heavier  (or  lighter)  load  is  sub- 
sequently lifted,  insufficient  (or  excessive)  muscular  force  is 
applied.  This  occurs  in  spite  of  the  recognition,  through 
touch  or  vision,  of  the  altered  size  of  the  load.  The 
load  is  therefore  judged  to  be  heavier  (or  lighter)  than  it 
really  is. 

Another  form  of  adaptation  is  instanced  in  the  effects 
of  continuously  wearing  glasses  which  invert  the  field  of 
vision.  In  course  of  time,  the  subject's  movements  become 
adapted  to  the  changed  position  of  seen  objects.  The 
results  of  such  adaptation  are  very  conspicuous  when  at 
length  the  glasses  are  discarded. 

In  the  ear,  adaptation  of  a  still  higher  order  is  met  with, 
which,  like  the  strictly  sensory  form,  masks  the  fatiguing 
effects  of  sensory  activity  on  consciousness.  A  tone  may 
be  sounded  incessantly  for  hours,  and  yet  it  remains  audible. 
To  such  a  continuous  auditory  stimulus  we  become  adapted ; 
ultimately  we  cease  to  notice  it.  But  we  may  become 
aware  of  it  at  any  moment  through  a  passive  or  voluntary 
change  in  attention.] 

BIBLIOGRAPHY. 

H.  Zwaardemaker,  Die  Physiologic  d.  Geruchs,  Leipzig,  1895,  78.  W. 
McDougall,  Reports  of  the  Cambridge  Anthropol.  Expedition  to  the  Torres 


254  EXPERIMENTAL  PSYCHOLOGY 

Straits,  Cambridge,  1901-3,  ii.  194.  C.  S.  Myers,  ibid.,  143  ;  W.  H.  R. 
Rivers,  ibid.,  13,  "Observations  on  the  Senses  of  the  Todas,"  Brit.  Journ. 
of  PsychoL,  1905,  i.  321.  E.  Toulouse,  N.  Vaschide  et  H.  Pieron,  Tech- 
nique de  Psychologic  Experimcntale,  Paris,  1904,  49.  B.  R.  Andrews, 
"Auditory  Tests,"  Amer.  Journ.  of  Psychol.,  1904,  xv.  14;  1905,  xvi. 
302. 


CHAPTEK    XIX 
ON  EXPERIENCES  OF  IDENTITY  AND  DIFFERENCE  * 

Weber  s  Law. — It  is  a  familiar  condition  that  two  stimuli 
must  differ  at  least  by  a  minimal  amount,  in  order  that  we 
may  become  aware  of  their  difference.  That  difference 
which,  throughout  a  long  series  of  trials,  turns  out  to  be  as 
often  appreciable  as  inappreciable,  we  have  termed  "the 
liminal  sjim^lus^difference  "  osfi  the  differential  threshold  of 
the  stimulus  "\  (pages  201,  210).  A  difference  which  exceeds 
this  liminal  value  becomes  more  often  appreciable  than 
inappreciable. 

If  i  be  the  magnitude  of  a  given  stimulus,  and  if  Ai 

Ai 

be  its  differential  threshold,  it  is  found  that  — s-    has  an 

i 

approximately  constant  value,  except  for  extremely  large  or 
extremely  small  values  of  J.  This  discovery  was  made  over 
seventy  years  ago  by  Weber,  when  he  was  determining  just 
appreciable  differences  between  certain  stimuli.  He  found 
that  a  practised  subject,  who  could  just  appreciate  the 
difference  between  lifted  weights  of  twenty-nine  and  thirty 
ounces,  could  also  just  distinguish  between  weights  of  twenty- 
nine  and  thirty  drams,  approximately.  He  arrived  at  similar 
conclusions  in  the  case  of  pressures  applied  to  the  skin,  and 
in  the  case  of  short  lines  compared  by  the  eye. 

In  more  general  terms,  he  concluded  that  the  just  appreci- 
able difference  between  two  objects  depends  on  the  ratio 
of  that  difference  to  the  magnitude  of  the  objects,  not  on 

1  See  footnote  to  Chapter  III. 

255 


256  EXPERIMENTAL  PSYCHOLOGY 

the  absolute  difference  between  the  magnitudes.  This  is 
Weber's  law,  which  has  since  been  found  to  hold  for  various 
other  forms  of  stimuli  besides  those  which  Weber  himself 
employed. 

The  law  is  but  an  exact  formulation  of  everyday  experi- 
ence. In  the  stillness  of  night  the  ticks  of  a  watch  are 
loud,  although  inaudible  amid  the  noise  of  a  railway  journey. 
A  candle,  carried  into  a  room  at  twilight,  makes  a  noticeable 
difference  in  its  illumination,  whereas  at  noon  the  effect  is 
inappreciable. 

The  relation  between  the  liminal  increment  Ai  and  the 
magnitude  of  the  stimulus  i  depends  upon  the  nature  of 
the  stimulus.  For  lifted  weights  it  is  as  we  shall  see  about 
V  one^thirtieth  (exp.  119).  For  pressures  on  the  finger-tip  it 
has  been  found  to  be  about  one-twentieth,  for  brightness  of 
light  about  one-hundredth,  and  for  intensities  of  noise  about 
one-third;  two  sounds  of  different  loudness  can  just  be 
distinguished  as  different,  provided  that  the  intensity 
of  one  is  greater  by  about  one-third  than  that  of  the 
other. 

Limits  of  the  Law. — The  law  is  only  obeyed  where  all 
disturbances  due  to  imperfect  sensory  adaptation  are 
eliminated.  For  example,  the  eye  must  be  adapted  to  the 
particular  brightness  value  of  i  which  is  chosen,  before  its 
differential  threshold  is  investigated ;  otherwise  the  ratio 

— r-  varies  for  different  values  of  i.     Further,  the  law  pre- 

i 

supposes  adequate  practice  on  the  part  of  the  subject  under 
investigation. 

Weber's  law  only  holds  for  moderate  values  of  stimulus 
strength.  If  we  can  just  detect  a  difference  between  weights 
of  twenty-nine  and  thirty  drams  or  ounces,  we  may  yet  be 
unable  to  distinguish  between  twenty-nine  and  thirty  stones 
or  grains ;  and  similarly  with  very  loud  or  weak  sounds,  or 
with  very  intense  or  weak  degrees  of  brightness.  Clearly, 
if  the  absolute  value  of  the  stimuli  become  sufficiently  small, 


IDENTITY  AND  DIFFERENCE  257 

a  limiting  point  is  reached  at  which  we  are  determining  no 
longer  the  differential  but  the  absolute  threshold. 

The  law  has  been  found  to  hold  for  differences  in  the 
intensity  of  odours  and  of  tones,  in  addition  to  the  above- 
named  stimuli.  The  law  broadly  holds  wherever  the  physio- 
logical bases,  i.e.  the  specific  nervous  energies,  underlying 
the  two  experiences,  differ  merely  in  degree.  On  the  other 
hand,  it  fails  where  the  bases,  or  specific  energies,  differ  in 
kind.  For  example,  the  smallest  appreciable  difference  of 
pitch  between  two  tones  is  constant  throughout  a  consider- 
able range  of  tones ;  a  person  who  can  just  distinguish  the 
tones  of  200  and  20075  vibrations  from  one  another  will 
just  be  able  to  distinguish  the  tones  of  400  and  40075 
vibrations.  Were  Weber's  law  operative,  a  difference  of  1*5 
instead  of  075  vibrations  would  be  necessary  in  the  latter 
case.  But  it  is  inoperative,  as  we  are  here  dealing  with  two 
different  kinds  of  specific  nervous  energy. 

The  Interrelation  of  Extensive  and  Intensive  Changes. — 
From  the  standpoint  of  physiology,  differences  in  spatial 
extent  are  not  so  far  removed  from  differences  in  intensity 
as  at  first  sight  they  appear  to  be.  In  the  first  place,  a 
given  nerve  fibre,  Na,  supplies  not  only  an  area  a  but  also 
helps  in  supplying  adjoining  areas  &,  c,  d ;  similarly  a  nerve 
fibre  Nc  supplies  not  only  c  but  also  the  areas  b  and  d,  and 
in  addition  perhaps  the  areas  a  and  e.  So,  when  the  areas 
a,  I,  c,  d,  e  are  simultaneously  stimulated,  the  physiological 
effect  on  the  fibre  Nc  is  similar  to  a  sufficiently  intense 
stimulation  of  the  area  c.  Secondly,  in  addition  to  this 
overlap  at  the  periphery,  neighbouring  nerve  fibres  are  often 
interconnected  more  centrally,  so  that  adequate  excitation 
of  a  nerve  cell  by  a  given  nerve  fibre  may  lead  to  overflow 
or  irradiation  of  the  impulse  into  other  cells  fed  by  other 
nerve  fibres.  For  these  reasons  it  is  not  surprising  that 
Weber's  law  holds  for  short  spatial  extents. 

Easily  Appreciable  Differences. — The  principle  involved 
in  Weber's  law  holds  not  only  for  just  appreciable,  but  also 


258  EXPERIMENTAL  PSYCHOLOGY 

for  easily  appreciable  differences  in  stimulus  magnitude.  If 
a  number  of  greys  of  different  brightness  be  so  chosen  as 
to  form  a  series  of  successive  equidistant  grades  of  brightness, 
then  the  ratio  of  the  physical,  i.e.  photometric,  intensities 
between  any  two  consecutive  greys  is  found  to  be  approxi- 
mately constant.  Thus,  if  eight  grey  papers,  a,  I,  c,  d,  e,  f,  g,  h, 
have  been  chosen  so  that  the  difference  between  c  and  6 
appears  equal  to  that  between  b  and  a,  the  difference  between 
d  and  c  appears  equal  to  that  between  c  and  b,  and  so  on  ; 
then  if  ia,  ib,  ic>  id  .  .  .  represent  the  physical  or  luminosity 

values  of  these  greys,  the  ratios  -A  -A  -£  -A  -/,  .  .  .  will  be 

*a   lb    lc    ld   % 

found  to  be  approximately  equal.  Astronomers  have 
obtained  like  results  in  classifying  the  stars  according  to 
apparently  equal  differences  of  magnitude.  Consequently, 
we  can  now  express  Weber's  law  in  more  general  terms  /like 
experiences  of  stimulus  difference  are  dependent  on  like 
relations  between  the  magnitude  of  the  difference  and  the 
absolute  magnitude  of  the  stimulus.} 

Fecliner's  Law.  —  After  having  more  exactly  verified 
Weber's  lawr  and  emphasised  its  importance,  Eechner 
proceeded  to  make  deductions  from  it,  by  the  help  of 
various  lines  of  mathematical  reasoning;  one  of  which  is 
here  given.  ^He  argued  that  if  sv  s2,  represent  two  sensa- 
tions produced  by  the  stimuli  ^  and  i2,  experiment  justified 

n 

him  in  concluding  that,  so  long  as  4   remains    constant, 

x  72 

sx—  s2  must  be  constant;)  in  other  words,  that 


or        *-«L-(         (2) 

where  f  is  the  usual   sign   of   functional   dependence   and 

implies  here  that   the   difference   between   two   sensations 
is    dependent    on    the    ratio    of    the    magnitudes    of   the 


IDENTITY  AND  DIFFERENCE  259 

stimuli.     Now  suppose  that  in  (1)  i'2  =  ^0,  being  a  strength 
of   stimulus  which   is   too   weak   to   produce   a   sensation. 


Then  s2  =  o  and 


*-        (3)    . 

Or  supposing  that  in  (2)  i±  =  iQ,  then  s1  =  o,  and 


From  tf)  and  (4)   »t-%=-( 

Hence  from  (1)     /()  =/($)-/($), 


but     /  (^)  may  be  expressed  as  /  (^  X  ^ 
v^  V 


This  is  an  equation  of  the  form 

/(«y)-/(« 

a  general  solution  of  which  is  only  possible  by  putting 


=A;logaj, 

and  f(y)    =  A;  logy, 
where  ^  is  a  constant. 

Therefore,         /($)  =  &  ]og$ 

and  (1)  becomes 


Let  us  now  suppose  s2  to  be  a  just  inappreciable  sensation, 

'&* 


260  EXPERIMENTAL  PSYCHOLOGY 

and  the  necessary  strength  of  the  corresponding  stimulus  i2 
to  be  equal  to  unity. 

Then  s1  =  k  log  i  ;  or  generally, 

S=JTlog  I. 

This  conclusion,  that  the  sensation  is  proportional  in 
strength  to  the  logarithm  of  the  stimulus,  constitutes 
Fechner's  law. 

[A  Critical  Examination  of  Fechner's  Law. — It  is  interesting 
to  follow  the  results  of  other  substitutions  in  the  equation 

• 

s  —s»  =  k  log  -A      Supposing   the  stimuli  to   be  of*  equal 

h 

strength,  then  tx  =  iz  and  k  log  -^  consequently  becomes  zero. 

h 
That  is  to  say,  s1  =  s2,  as  is  truly  the  case. 

On  the  other  hand,  supposing  that  ^  is  ever  so  slightly 
different  from  i2,  then  sx  and  s2  must  immediately  differ. 
But  this  is  quite  contrary  to  actual  experience.  The  expres- 
sion Sj  —  s2  remains  zero  until  -£  has  exceeded  a  certain  liminal 

value. 

Again  when,  in  the  equation  S=K  log  /,  /  is  unit 
strength,  S  becomes  zero  as  it  should  by  hypothesis.  But 
how  are  we  to  interpret  the  increasing  negative  values  of  S, 
which  are  given  as  the  strength  of  /  diminishes  from  unity 
to  zero  ? 

Endeavours  have  been  made  to  escape  from  the  former, 
at  least,  of  these  difficulties,  by  distinguishing  between  the 
physiological  excitation  on  the  one  hand,  and  the  experience 
of  sensation  on  the  other :  between  the  activity  of  the 
lower  parts  of  the  nervous  system,  which  form  the  sensory 
apparatus,  and  the  activity  of  the  higher  cerebral  centres, 
whereby  we  become  conscious  of  sensations  and  sensation 
differences. 

'  We  must  admit  that,  when  a  subliminal  stimulus  or  a 
subliminal  stimulus  difference  produces  no  change  in  con- 
sciousness, it  may  nevertheless  have  a  non-conscious,  solely 


IDENTITY  AND  DIFFERENCE  261 

physiological,  action  of  some  kind.  And  it  is  quite  conceiv- 
able that  it  is  the  relation  between  this  purely  physiological 
action  and  the  strength  of  the  stimulus  that  Fechner's  law 
expresses.  But  Fechner  maintained  that  the  logarithmic 
relation,  implied  in  his  law,  lay,  not  between  these,  but 
between  the  physiological  action  (or  psycho-physical  activity, 
as  he  termed  it)  and  the  mental  experience  of  the  sensation  ; 
he  believed  that  the  law  expressed  the  relation  between 
"  psycho-physical  "  and  "  mental  "  process. 

Within  the  limits  of  an  elementary  treatise,  it  is  impossible 
to  examine  Fechner's  other  views  in  adequate  detail.  But  it 
should  be  noticed  that  he  regarded  a  sensation  as  the  sum  of 
a  number  of  just  appreciable  unit  increments  of  sensation,  or 
as  the  sum  of  a  number  of  just  appreciable  unit  sensation 
differences.  He  maintained  that  the  change  of  sensation, 
obtained  by  adding  one  ounce  to  a  weight  of  twenty-nine 
ounces,  was  absolutely,  as  well  as  relatively,  the  same  as 
that  obtained  by  adding  one  drain  to  a  weight  of  twenty- 
nine  drams.  Of  course,  were  this  so,  an  ounce  and  a  dram 
should  produce  an  equal  sensation. 

Fechner's  error  lay  in  his  rigid,  unreflecting  application 
of  mathematics  to  psychological  data.  By  the  equation 

s1  —  s2=  /  (^)  we  mean  that  our   experience  of   difference 

between  two  stimuli  is  dependent  on  the  ratio  of  the  magni- 
tudes of  the  stimuli.  Experiment  teaches  us  that  it  holds 
only  for  moderate  values  of  ^  and  iz.  Hence  when  %  is  so 
small  (  =  i0)  that  it  fails  altogether  to  evoke  any  experience 

at  all,  it  is  quite  unjustifiable  to  conclude  that  sl  =ffe\  or 


similarly  that  s2  =  . 

J  VV 

Moreover,  Fechner  assumed  that  the  expression  s1  —  s2 
stands  for  a  difference  between  two  experiences,  whereas 
it  denotes  an  "experience  of  difference."  The  experience 
that  two  stimuli  are  identical  or  different  as  strictly  sui 


262  EXPERIMENTAL  PSYCHOLOGY 

generis  as  the  experience  of  a  stimulus  itself.  Indeed,  we 
may  go  still  further,  and  say  that  our  judgment,  that  an 
experience  of  one  difference  is  identical  with  or  different 
from  another,  is  as  single  and  undivided  an  experience  as 
our  judgment,  that  an  experience  of  one  stimulus  is  identical 
with  or  different  from  another.  In  each  instance,  what  is  in 
consciousness  is  an  experience  of  identity  or  of  difference. 
The  threshold  for  the  experience  of  a  difference  demands 
precisely  the  same  psychological  treatment  as  is  accorded  to 
the  threshold  for  the  experience  of  a  stimulus. 

For  this  reason  we  are  not  warranted  in  treating  the  ex- 
pression s1  —  s2  as  if  it  expressed  merely  a  difference  between 
two  experiences,  and  in  adding  to  it,  subtracting  from  it,  or 
otherwise  manipulating  it  as  if  two  separate  psychic  entities 
were  present.  The  expression  s1  —  s2  represents  a  single  state 
of  consciousness,  the  experience  of  a  difference.  It  admits 
neither  of  dissection  nor  of  mathematical  treatment.  --m 

This  distinction  which  we  have  drawn  between(a  differ- 
ence of  experience^  and  an/experience  of  a  difference)  is  well 
brought  out,  when  we  consider  the  case  of  two  stimuli  which 
are  not  sufficiently  different  to  give  rise  to  an  experience  of 
difference.  Because  we  have  here  a  subliminal  experience 
of  difference,  it  would  be  wrong  to  conclude  that  sx  and  s2, 
the  psychic  effects  of  the  two  stimuli,  are  in  themselves 
equal.  For  if  s±  and  s2  were  identical  in  the  case  of  two 
subliminally  different  stimuli  ^  and  i2,  and  if  similarly  s2 
and  s3  were  identical  in  the  case  of  two  subliminary  stimuli, 
i2  and  i3 ;  then,  in  the  case  of  the  two  stimuli  i:  and  i3,  s1 
and  s3  should  be  identical,  whereas  in  fact  there  may  be  a 
very  distinct  experience  of  difference. 

Lastly,  even  if  sx  were  equal  toy  (M  and  s2  to 
Fechner's    conclusions    that   s1  —  s2   is    equal 

^)  would  be  psychologically  unjustifiable.     A  moment's 
V 


IDENTITY  AND  DIFFERENCE  263 

reflection  will  show  that  we  cannot  manipulate  our  sensa- 
tions in  this  way.  For  instance,  we  cannot  say  that  we 
double  the  strength  of  a  sensation  when  we  superimpose 
two  sensations  of  equal  strength  upon  one  another.] 

[Immeasurability  of  Sensation. — Indeed  we  are  powerless 
to  measure  sensation  strengths  at  all.  We  can  say  that  one 
sensation  is  equal  to,  greater  or  less  than,  another  sensation 
produced  by  a  stimulus  of  like  or  different  magnitude,  but 
we  cannot  say  how  much  the  one  is  greater  or  less  than  the 
other.  The  cannon  roar  is  louder  than  the  crack  of  a  pistol, 
but  we  cannot  express  the  former  experience  in  terms  of  the 
latter. 

A  quantity  is  only  measurable  when  it  is  capable  of 
division  into  equal  unit  parts.  Temporal  and  spatial  experi- 
ences are  thus  divisible.  We  can  measure  the  intensity  of 
a  visual  or  an  auditory  stimulus  by  the  extent  or  amplitude 
of  the  vibration ;  we  can  measure  a  given  weight,  a  given 
size,  or  a  given  time  interval  by  the  number  of  molar, 
spatial,  or  temporal  units  which  it  contains.  Sensations, 
however,  qud  sensations,  cannot  thus  be  measured.  They 
can  be  arranged  in  a  graded  series,  increasing  or  decreasing 
in  intensity,  but  we  cannot  divide  a  sensation  into  equal 
unit  sensations.  All  that  we  can  say  is  that  so  many  pistols 
simultaneously  fired  would  produce  the  sensation  of  a  given 
cannon  roar,  that  so  many  candles  simultaneously  lighted 
would  produce  the  sensation  of  a  given  arc  light,  that  so 
many  ounce  weights  simultaneously  lifted  would  produce 
the  sensation  of  a  given  pound  weight. 

But  to  state  the  conditions  under  which  a  given  sensa- 
tion may  result  is  not  equivalent  to  measuring  the  strength 
of  a  sensation  itself ;  it  is  not  equivalent  to  stating  that  one 
sensation  is  a  hundredfold  or  a  hundredth  part  of  another 
or  unit  sensation.  From  the  standpoint  of  experience 
such  sensations  have  no  numerical  relation  to  one  another. 
Consequently,  the  determination  of  their  magnitude  by 
reference  to  the  strengths  or  extents  of  the  corresponding 


264  EXPERIMENTAL  PSYCHOLOGY 

stimuli  becomes  meaningless.     In  other  words,  Fechner's 
law  has  no  psychological  foundation.] 

The  Basis  of  Weber's  Law. — We  have  seen  that  Fechner 
believed  that  Weber's  law  was  an  expression  of  the  relation 
between  the  physiological  excitation  due  to  sensory  stimula- 
tion, on  the  one  hand,  and  its  affection  of  consciousness  on 
the  other.  But  other  interpi^tatioriFoOEe  lawTTave  been 
offered,  which  it  is  now  our  duty  to  examine. 

Wundt  has  suggested  that  the  process  of  "  apperception  " 
is  the  cause  of  Weber's  law.  He  supposes  ~that~a  given 
stimulus  or  stimulus  difference  can  only  be  adequately 
experienced  after  the  more  elementary  psychical  reactions 
to  which  it  gives  rise  have  been  subject  to  apperception,  by 
being  related  to  other  experiences.  But  if  it  be  in  this 
higher  ".faculty"  of  apperceptive  comparison  that  Weber's 
law  originates,  we  may  well  ask  why  the  law  does  not 
hold  in  the  case  of  all  judgments  of  difference, — for  example, 
in  the  case  of  the  estimation  of  the  smallest  appreciable 
differences  of  pitch  ? 

It  is  far  more  probable  that  the  law  is  a  general  expres- 
sion of  the  relation  of  protoplasmic  activity  to  the  strength 
of  the  exciting  stimulus.  For  if  the  extent  of  negative 
variation  of  the  current  be  observed  in  a  stimulated  afferent 
nerve,  and  if  it  may  be  regarded  as  evidence  of  the  strength 
of  the  nervous  impulse,  we  appear  to  be  face  to  face  with 
Weber's  law,  when  the  frog's  eye  is  stimulated  by  lights 
of  different  intensity,  or  when  the  frog's  skin  is  stimulated 
by  weights  falling  on  it  with  different  momenta. 

Ebbinghaus  suggests  that  the  law  may  be  a  result  of  the 
difference  in  degree  of  stability  between  the  protoplasmic 
molecules  which  are  broken  down  by  the  stimulus.  The 
more  unstable  molecules  are  easily  decomposed  by  relatively 
feeble  stimuli.  Thus,  when  two  weak  stimuli  follow  one 
another,  a  small  difference  between  them  is  sufficient  to 
produce  a  difference  in  the  number  of  molecules  upon  which 
they  have  acted.  On  the  other  hand,  when  two  strong 


IDENTITY  AND  DIFFERENCE  265 

stimuli  follow  one  another,  the  less  stable  molecules  are 
decomposed  by  the  first  strong  stimulus,  and  consequently 
a  greater  difference  between  the  two  stimuli  is  necessary  for 
the  one  stimulus  to  succeed  in  breaking  down  a  greater  or 
less  number  of  molecules  than  the  other.  If  the  sensation 
difference  depend  on  such  differences  of  molecular  decom- 
position, it  is  held  that  we  have  a  rough  diagrammatic  con- 
ception of  the  mode  of  action  of  Weber's  law. 

It  has  also  been  suggested  that  the  law  is  due  to  the 
extent  of  irradiation  of  the  given  impulse  to  neighbouring 
ganglion  cells ;  the  area  of  irradiation  being  relatively  less, 
the  greater  the  strength  of  the  stimulus. 

The  Completeness  of  the  Judgment. — So  far  we  have  con- 
fined our  attention  to  the  nature  and  significance  of  Weber's 
law,  as  if  our  judgments  of  identity  and  of  difference  were 
due  solely  to  its  operation.  The  remaining  pages  of  this 
chapter  will  show  that  this  is  very  far  from  being  the  case. 

The  threshold  of  difference  is  dependent  on  the  com- 
pleteness of  the  judgment  that  is  required.  The  threshold 
is  generally  lower  when  the  subject  is  merely  asked  whether 
or  not  a  difference  exists  than  when  he  is  asked  to  deter- 
mine the  direction  of  that  difference. 

[The  Mode  of  Presentation. — The  delicacy  of  discrimina- 
tion also  depends  upon  whether  the  two  stimuli  are  pre- 
sented simultaneously  or  successively.  The  threshold  is 
usually  lower  for  successive  than  for  simultaneous  presenta- 
tions. For  example,  smaller  differences  of  weight  can  be 
discriminated  when  the  objects  are  lifted  successively  than 
when  they  are  lifted  simultaneously.  Moreover,  when  they 
are  successively  lifted  by  the  same  hand,  the  threshold  is 
lower  than  when  they  are  lifted  by  different  hands. 

The  appreciation  of  difference  also  depends  on  the 
suddenness  with  which  the  change  is  made  from  the  one 
stimulus  to  the  other.  The  differential  threshold  may  be 
enormously  raised,  if  only  the  increase  or  decrease  of  the 
stimulus  proceed  with  adequate  slowness  and  regularity. 


266  EXPERIMENTAL  PSYCHOLOGY 

On  the  other  hand,  beyond  certain  limits,  a  too  rapid  change 
of  a  continuously  varying  stimulus  causes  a  rise  in  the 
differential  threshold.] 

[Practice. — Practice  is  of  considerable  influence  in  lower- 
ing the  differential  threshold.  Its  effects  are  confined  within 
surprisingly  narrow  limits  to  the  particular  exercise  per- 
formed. Thus  the  practice,  acquired  in  discriminating  suc- 
cessive sounds  of  nearly  identical  pitch,  is  largely  lost  when 
the  subject  is  confronted  with  sounds  of  different  timbre 
produced  by  another  instrument,  or  when  one  region  of  a 
scale  is  exchanged  for  another.  Similarly,  persons  who  are 
extremely  sensitive  to  differences  of  timbre  (e.g.  to  differ- 
ences of  voice)  may  have  very  obtuse  powers  of  discrimina- 
tion in  regard  to  differences  of  pitch.  A  threshold  which 
practice  has  lowered  on  one  side  of  the  body  is  almost 
correspondingly  lowered  on  the  symmetrically  opposite  side 
of  the  body.] 

The  Time  Error. — It  has  been  found  that  when  pairs  of 
heavy  weights  are  lifted,  the  second  of  any  pair  tends  to  be 
judged  heavier  than  the  first.  This  is  Fechner's  negative 
time  error.  It  is  said  to  be  increased  by  slow  lifting  and  by 
fatigue,  and  to  be  decreased  when  the  weights  are  light. 
With  light  weights  the  time  error  may  be  positive.  We 
may  explain  the  positive  time  error  by  supposing  that 
the  nervous  impulses,  involved  in  the  first  lift,  favourably 
influence,  either  by  facilitation  or  by  incitation,  the  strength 
of  the  next  nervous  impulses  corresponding  to  the  second 
lift.  In  equating  long  lines  and  short  lines,  the  time  error  is 
found  to  change  in  a  sense  opposite  to  that  which  holds  for 
lifting  weights.  But  individual  variations,  and  variations  of 
the  same  individual  under  different  conditions,  show  that  the 
causes  that  influence  the  time  error  are  far  too  complex  to 
be  explained  at  present  (exps.  120,  121). 

The  Time  Interval. — The  recognition  of  identity  or  differ- 
ence between  two  stimuli  is  much  affected  by  the  interval  of 
time  which  elapses  between  their  presentation.  When  the 


IDENTITY  AND  DIFFERENCE  267 

individual  has  had  adequate  practice,  and  when  the  interval 
between  the  two  stimuli  is  sufficiently  short,  he  tends  to 
give  the  judgment  of  difference  or  identity  immediately  and 
unreflectingly.  Introspection  shows  that  under  these  con- 
ditions he  often  makes  no  true  comparison  between  them. 
He  does  not  hold  the  one  stimulus  in  his  mind  beside  the 
other,  passing  from  the  first  to  the  second  and  from  the 
second  back  to  the  first,  weighing,  so  to  speak,  the  difference 
between  them.  On  the  contrary,  his  decision  is  immediate, 
and  involves  no  comparison.  It  is  as  if  the  one  presentation 
produced  one  change  in  the  total  mental  attitude  or  dis- 
position of  the  subject,  while  the  other  created  another 
change,  and  as  if  the  judgment  of  identity  or  difference  were 
the  unreflecting  result  of  sameness  or  want  of  sameness 
between  these  two  reactions.  In  such  situation  the  judgment 
rests  on  a  vague  state  remote  from  clear  cognition, — on 
what  we  loosely,  perhaps  wrongly,  term  a  "  feeling "  of 
familiarity  or  unfamiliarity  (page  150). 

With  increasing  interval  of  time  between  the  two  pre- 
sentations, especially  in  the  absence  of  practice,  memory 
images  of  the  two  stimuli  may  become  an  important,  if  not 
an  essential,  factor  in  the  judgments  of  identity  or  difference. 
The  vividness  alnd  accuracy  with  which  some  kind  of  memory 
image  of  the  first  stimulus  can  be  evoked  upon,  or  just  after, 
the  presentation  of  the  second  stimulus,  may  materially 
affect  the  correctness  of  the  subject's  answers  (page  149). 

The  Absohite  Impression. — A  further  complication  is 
especially  liable  to  arise,  when,  as  so  often  happens  during 
a  series  of  experiments,  one  of  the  stimuli  is  constant,  or 
when  all  the  stimuli  are  taken  from  a  narrow  region  of  their 
possible  range.  The  subject  comes  to  form  an  "absolute 
impression"  of  the  given  stimuli.  His  judgments  no  longer 
rest  simply  on  the  comparison  of  one  stimulus  with  another, 
but  they  are  influenced  by  the  absolute  impression  conveyed 
by  one  or  other  or  both  stimuli ;  just  as  in  everyday  life  we 
speak  of  to-day  as  bright  or  dull,  or  of  a  given  sound  as  loud 


268  EXPERIMENTAL  PSYCHOLOGY 

or  faint,  without  going  through  the  process  of  comparing  it 
with  other  days  or  with  other  sounds. 

The  influence  of  the  absolute  impression  comes  prom- 
inently to  the  fore  in  such  an  investigation  as  the  following, 
in  which  the  constant  method  (the  method  of  right  and 
wrong  cases)  is  applied  to  the  discrimination  between  a 
standard  and  a  variable  weight  placed  simultaneously  before 
the  subject  and  successively  lifted  by  his  same  hand.  The 
research  is  worth  a  detailed  description  for  the  light  that  it 
throws  on  the  value  of  a  psychological  experiment,  methodi- 
cally conducted  by  the  experimenter  and  adequately  assisted 
by  introspection  on  the  part  of  the  subject  (exp.  121). 

We  shall  call  the  standard  weight  S  and  one  of  the 
variable  weights  S—  d.  Outwardly,  the  two  weights  are 
precisely  alike,  and  the  conditions  of  lifting  each  weight 
are  as  nearly  as  possible  constant;  a  metronome  and  a 
horizontal  piece  of  string  regulating  the  rate  and  the  height 
of  lifting  successive  weights.  Then  for  the  two  weights 
S  and  S—  d,  there  are  the  following  four  possible  relations 
of  the  two  lifts  in  space  and  time : — 

av  standard  weight  lifted  first  and  placed  to  the  right  of 

the  variable, 

az,  „  „  second  and  placed  to  the  right 

of  the  variable, 

«3,  „  „  first  and  placed  to  the  left  of 

the  variable, 

a4,  „  „  second  and  placed  to  the  left 

of  the  variable. 

Throughout  every  series  of  investigations  we  shall 
employ  not  only  the  pair  of  weights  (S  and  S—d),  for  which 
we  have  used  the  letter  a,  but  also  the  pair  of  weights 
($and  S+d),  which  we  shall  denote  by  the  letter  I.  This 
latter  pair  may  be  lifted  in  any  one  of  the  temporal  or 
spatial  relations  bv  12,  Z>3,  Z>4,  corresponding  to  av  a2,  «3,  a4  of 
the  former  pair.  Thus  there  are  eight  separate  heads  under 
which  the  percentage  of  right  and  wrong  answers  may  be 


IDENTITY  AND  DIFFERENCE  269 

classified,  according  to  the  different  relations  of  the  standard 
and  variable  weights.  We  shall  use  the  letters  al  —  «4, 
&!  — &4,  briefly  to  express  the  different  percentages  of  right 
answers  obtained  under  these  eight  heads.  The  subject  is 
directed  to  return  his  answers  in  terms  of  the  second  pre- 
sentation, e.g.  "  second  lighter,"  "  second  heavier,"  "  no  differ- 
ence." 

Excepting  the  slight  effect  due  to  the  operation  of 
Weber's  law,  we  should  expect  that  2a  =  26,  where  20,  =  a±  + 
&2  +  a34-&4,  and  2&  =  \  +  &2  +  &3  +  &4. 

With  the  same  limitations,  we  should  also  expect  that 
«]_  =  &4 ;  in  other  words,  that  the  percentage  of  right  answers, 
when  the  heavier  weight  is  lifted  first  and  lies  to  the  right 
of  the  second  lifted  weight,  should  be  the  same  for  S  and 
S—  d  as  it  is  for  S  and  S+  d.  Similarly,  we  should  expect 
that  &2  =  53,  a3  =  &2,  and  &4  =  &r 

Instead  of  this,  we  find  from  experiment  that  as  a  rule 
a1>&4,  and  that  &2<&3,  ^3>&2,  a±<\-  We  conclude  that  the 
percentage  of  right  answers  is  greater  when  the  standard 
precedes  than  when  it  follows  the  variable.  This  has  been 
called  the  "  general  tendency  of  judgment." 

Further,  the  equation  2&=2&  usually  does  not  hold. 
Some  individuals  return  a  sensibly  greater  percentage  of 
right  answers  when  the  standard  is  heavier  than  the 
variable  (2&>2&),  while  others  succeed  better  when  the 
standard  is  lighter  than  the  variable  (2&<  2&).  The  former 
are  spoken  of  as  belonging  to  the  positive  type,  the  latter  to 
the  negative  type,  while  those  for  whom  2&  =  2&  belong  to 
the  indifferent  type.  We  may  call  this  condition  the 
"  typic  tendency 'of  judgment." 

The  effects  of  time  and  space  order  may  be  also  in- 
vestigated. Summing  a±  and  as,  we  have  the  effect  of  the 
standard  lifted  first ;  summing  #2  and  a4,  we  have  the  effect 
of  the  standard  lifted  second.  Let  us  call  the  percentage 
for  these  two  time  orders  A1  and  A2,  and  those  for  the 
corresponding  two  time  orders  of  the  "b  series  B^  and  B2. 


270  EXPERIMENTAL  PSYCHOLOGY 

Then,  in  the  absence  of  other  influences,  we  should  expect 
the  values  Av  Bv  A2,  B2  to  be  equal.  If  Al  is  greater  or 
less  than  A2,  and  Bx  is  similarly  less  or  greater  than  B%,  we 
must  attribute  their  difference  to  the  time  error,  and 
estimate  it  in  the  manner  already  indicated.  The  time 
error  (page  266)  is  conveniently  termed  positive  or  negative 
according  as  the  influence  of  the  time  order  causes  the  first 
presentation  to  appear  greater  or  less  than  the  second.  The 
space  error  may  be  similarly  determined  (page  203). 

Martin  and  Mtiller,  to  whom  we  owe  this  research,  have 
applied  their  investigations  to  so  many  individuals,  and 
have  extended  their  experiments  over  so  long  a  time,  that 
the  utmost  reliance  can  be  placed  on  the  outcome  of  their 
accumulated  data.  They  suggest  that  the  general  tendency 
of  judgment  («1>&4,  ^2<^3'  a3>&2»  a4>A)  an(i  the  typic 


tendency  of  judgment  (2^=2^)  receive  a  ready  explanation, 

if  the  influence  of  the  "  absolute  impression  "  be  taken  into 
consideration.  When,  during  a  series  of  continuous  experi- 
ments, one  and  the  same  standard  weight  is  used,  we  come 
to  judge  of  the  lifted  weight  as  we  judge  of  weights  in 
ordinary  life,  e.g.  the  weight  of  a  box  as  absolutely  heavy, 
or  the  weight  of  a  baby  as  absolutely  light  ;  and  our  verdict 
comes  to  be  much  influenced  by  this  absolute  impression. 
Martin  and  Mliller  conclude  that  this  absolute  impression 
of  heaviness  or  lightness  enters  more  frequently  in  the  case 
of  weights  which  are  heavier  or  lighter  than  the  standard 
weight  than  in  the  case  of  the  standard  weight  itself  ;  and 
it  would  seem  that  the  greater  the  difference  between  the 
variable  and  the  standard,  the  more  evident  is  the  action  of 
the  absolute  impression.  While  admitting  that  in  some 
cases  our  judgment  of  the  difference  between  two  lifted 
weights  depends  on  actual  comparison  between  them,  they 
insist  that  we  often  form  an  absolute  impression  of  one  of 
the  weights,  so  that,  if  the  first  or  second  lifted  weight  give 


IDENTITY  AND  DIFFERENCE  271 

the  impression  of  absolute  lightness  or  heaviness,  the  other 
weight  tends  to  be  pronounced  the  heavier  or  the  lighter. 

Now  the  absolute  impression  of  the  first  lifted  weight 
has  obviously  less  effect  on  the  judgment  than  the  absolute 
impression  of  the  second  lifted  weight,  since  it  can  only 
influence  the  judgment  by  being  remembered.  If  we  add 
to  this  the  already  mentioned  condition  that  the  variable 
awakens  an  absolute  impression  more  often  than  the 
standard,  we  account  for  those  results  which  Martin  and 
Muller  describe  as  the  general  tendency  of  judgment. 

The  positive,  negative,  and  indifferent  types  of  judgment 
appear  to  be  chiefly  dependent  on  the  strength  of  the 
subjects.  Most  men,  being  powerful  lifters,  conform  to  the 
positive  type ;  that  is  to  say,  they  return  more  right  answers 
when  the  variable  is  less  than  when  it  is  greater  than  the 
standard.  Most  women,  on  the  other  hand,  seem  to  belong 
to  the  negative  type.  Occasionally  the  type  of  a  subject 
changes  during  a  long  continued  course  of  experiment, 
passing  through  a  stage  of  indifference  (2#  =  2&)  from  one 
type  to  the  other.  The  onset  of  fatigue,  or  the  increase  in 
weight  of  the  standard  weight,  leads  to  an  increase  of 
negativity  in  a  subject  of  the  negative  type,  and  to  a 
decrease  of  positivity,  or  even  to  reversal,  in  a  subject  of 
the  positive  type. 

According  to  Martin  and  Muller,  these 'individual  differ- 
ences and  changes  of  type  are  the  natural  results  of  the 
individual  differences  and  changes  in  absolute  impression. 
A  person  who  with  given  weights  forms  the  absolute 
impression  of  lightness,  will  appreciate  a  weight  of  increased 
lightness  more  readily  than  one  of  increased  heaviness ; 
whereas  a  more  weakly  person,  who  with  the  same  weights 
forms  the  absolute  impression  of  heaviness,  will  appreciate 
more  readily  a  weight  of  increased  heaviness  than  one  of 
increased  lightness.  Such  an  explanation  receives  some 
confirmation  from  observation  of  the  ways  in  which  different 
subjects  express  their  replies,  during  the  course  of  a  special 


272  EXPERIMENTAL  PSYCHOLOGY 

series  of  experiments  in  which  the  subject's  direction  of 
judgment  is  left  entirely  free  (page  216).  One  subject,  for 
example,  who  is  manifestly  of  the  positive  type,  is  found 
always  to  give  his  answers  in  terms  of  lightness,  e.g. 
"  right  weight  lighter,"  "  left  weight  lighter."  The  contrary 
replies  occur  in  subjects  of  the  negative  type,  whereas  in  a 
subject  of  the  indifferent,  his  answers  are  given  in  various 
directions. 

Side  Comparisons.  —  The  experimental  data  and  the 
introspective  records  will  also  show  the  occasional  influence 
of  "side  comparisons."  That  is  to  say,  the  subject's 
judgment  is  influenced  by  one  or  other  of  the  stimuli  of 
the  preceding  pair.  If,  for  example,  the  first  of  a  given 
pair  of  stimuli  appear  smaller  than  a  preceding  stimulus, 
it  is  apt  thereby  to  appear  smaller  than  the  second 
stimulus  with  which  it  has  to  be  compared. 

[  The  Coherence  of  Stimuli. — We  are  now  in  a  position  to 
appreciate  the  vast  number  of  different  influences  which 
tend  to  obscure  and  to  counteract  the  play  of  Weber's  law. 
No  wonder,  then,  that  the  latter  is  especially  apt  to  be 
over-ridden  in  the  determination  of  equal-appearing  differ- 
ences (page  215).  Indeed,  it  is  sometimes  found  that  the 
stimulus  b,  so  far  from  being  the  geometrical  mean  between 
a  and  c,  approaches  or  even  exceeds  the  arithmetical  mean, 
when  the  subject  judges  that  the  difference  between  a  and  I 
is  equal  to  that  between  b  and  c.  This  divergence  from 
Weber's  law  probably  occurs  when  the  presentations  corre- 
sponding to  a  and  b  or  to  c  and  d  are  incapable  of  being 
apprehended  as  a  single  experience.  So  long  as  the  subject 
is  engaged  in  equating  two  single  experiences,  the  law  is 
approximately  obeyed.  For  various  reasons,  however,  a  and 
b,  or  b  and  c,  may  not  "  cohere  "  so  as  to  form  such  a  single 
experience.  If,  for  instance,  a  be  very  bright  or  very  heavy, 
the  attention  is  apt  to  be  directed  specifically  towards  it,  an 
absolute  impression  of  brightness  or  heaviness  results,  and, 
in  consequence,  a  and  b  never  blend  sufficiently,  but  remain 


IDENTITY  AND  DIFFERENCE  273 

more  or  less  isolated  from  one  another  (exp.  122).  A 
similar  explanation  may  account  for  the  fact  that,  while 
(under  favourable  conditions)  Weber's  law  holds  good  for  the 
differential  threshold  of  intervals  of  time,  it  does  not  hold 
good  when  a  time  interval  is  sought  which  lies  midway  in 
length  between  two  other  time  intervals.] 


BIBLIOGRAPHY. 

E.  H.  Weber,  "Der  Tastsinn  u.  d.  Gemeingefiihl, "  Wagner's  Hand- 
worterbuchd.  PhysioL,  Braunschweig,  1846,  iii.  2te  Abth.,  481.  G.  T. 
Fechner,  Elemente  d.  Psychophysik,  Leipzig,  1860.  E.  Hering,  "Zur  Lehre 
v.  d.  Beziehung  zw.  Leib  u.  Seele,"  Sitzungsber.  Wiener  Akad.  d.  Wiss., 
Math.-nat.  01.,  1875,  Ixxii.  Abth.  3,  310.  H.  Ebbinghaus,  "Uber  d. 
Grand  d.  Abweichungen  v.  d.  Weber'schen  Gesetz  bei  Lichtempfindungen," 
Arch.f.  d.  ges.  PhysioL,  1889,  xlv.  113.  A.  D.  Waller,  "Points  relating  to 
the  Weber-Fechner  Law,"  Brain,  1895,  xviii.  200.  L.  J.  Martin  u.  G.  E. 
Miiller,  Zur  Analyse  d.  UnterscTiiedsempfindlidikeit,  Leipzig,  1899.  F. 
Angell  and  G.  Harwood, "  Discrimination  of  Clangs  for  Different  Intervals  of 
Time,"  Amer.  Journ.  of  PsychoL,  1899,  xi.  67  ;  1900,  xii.  58.  W.  Ainent, 
"  tiber  d.  Verhaltniss  d.  ebenraerklichen  zu  d.  iiberraerklichen  Unter- 
schieden  bei  Lichtu.  Schallintensitaten,"  Philosoph.  Stud.,  1900,xvi.  135.  F. 
Angell,  "  Discriminations  of  Clangs  for  Different  Intervals  of  Time,"  Amer. 
Journ.  of  PsychoL,  1901,  xii.;  "Discriminations  of  Shades  of  Grey  for 
Different  Intervals  of  Time,"  Philosoph.  Stud.,  1902,  xix.  1.  W.  Wundt, 
Grundziige  d.  physiol.  PsychoL,  Leipzig,  1902,  i.  493.  J.  Frobes,  "  Ein 
Beitrag  liber  die  sogenannten  Vergleiclmngen  libermerklicher  Empfind- 
ungsunterschiede,"  Ztsch.  f.  PsychoL  u.  PhysioL  d.  Sinnesorgane,  1904, 
xxxvi.  241,  344.  E.  B.  Titchener,  Experimental  Psychology,  New  York, 
1905,  ii.  introduction  to  pts.  1,  2.  A.  Aliotta,  La  Misura  in  Psicologia 
Sperimentale,  Firenze,  1905. 


18 


CHAPTEK  XX 
ON   BINOCULAR  EXPERIENCE1 

Binocular  Combination. — The  images  of  two  like  objects, 
received  separately  by  the  two  retinae,  may  combine  to 
produce  vision  of  a  single  object.  Such  binocular  combina- 
tion may  almost  as  easily  be  produced  without  the  aid  of 
the  stereoscope  as  with  it  (exps.  123,  124). 

Corresponding  and  Disparate  Eetinal  Points. — Those  twin 
points,  one  on  one  retina,  the  other  on  the  other,  which  ascribe 
the  same  localisation  to  an  object  in  the  field  of  binocular 
vision,  are  termed  "  corresponding  "  points.  When  the  rays 
from  an  object  fall  on  corresponding  points  of  the  retinae, 
the  object  is  seen  as  a  single  object.  When  the  rays  fall  on 
non-corresponding,  or,  as  they  are  termed,  on  "  disparate  " 
points,  either  single  vision  or  "  diplopia  "  (i.e.  double  vision) 
results,  according  to  circumstances  into  which  we  shall 
presently  (page  276)  enter. 

Uncrossed  and  Crossed  Disparation.  —  When  a  single 
object  is  doubled  in  binocular  vision  (exp.  125),  the  image 
received  by  each  eye  appears  to  be  on  the  same  side  as,  or 
on  the  side  opposite  to,  the  eye  that  receives  it,  according  as 
the  object  lies  farther,  or  nearer,  than  the  point  of  fixation. 
These  conditions  of  "  uncrossed  "  and  "  crossed  "  images  are 
known  respectively  as  uncrossed  and  crossed  "  disparation." 

The  accompanying  diagram  (fig.  9)  more  fully  explains 
their  causation.  Here  F  is  the  fixation  point,  rays  from 
which  fall  on  the  corresponding  retinal  points  f^f?  0  is 

1  See  footnote  to  Chapter  III. 

274 


BINOCULAR  EXPERIENCE  275 

an  object,  lying  beyond  the  fixation  point.  Rays  from  it 
fall  on  the  markedly  disparate  retinal  points  ov  02.  In  con- 
sequence, diplopia  occurs,  the  two  images  of  the  object  o 
appearing  as  0'  and  0".  When  the  object  lies  nearer  than 
the  fixation  point,  as  at  X,  the  well-marked  disparation  of 
the  retinal  points  x1  and  x2  again  produces  diplopia,  but  in 
this  ease  the  disparation  is  crossed.  The  images  X'  and  X" 
are  treated  as  objects  situated  on  the  side  opposite  to  the 
eye  which  receives  them,  whereas  the  images  O  and  0"  are 
treated  as  objects  situated  on  the  same  side  as  that  of  the 
eye  which  receives  them. 

Covering  Points. — Corresponding  points  have  been  some- 
times termed  "  identical "  or 
"  covering  "  points.  But  it 
would  be  a  mistake  to  sup- 
pose that,  if  the  two  retinse 
were  superimposed,  with  the 
right  and  left  fovese  and 
the  horizontal  and  vertical 
axes  overlying,  the  corre- 
sponding points  would  then 
accurately  coincide.  Ex- 
periments have  shown  that  pIG  9 
between  the  vertical  meri- 
dians of  the  two  retina?  a  "  physiological  incongruence " 
exists.  When  the  eyes  are  in  the  "  primary  "  position,  i.e. 
when  they  are  horizontally  regarding  an  infinitely  distant 
object,  the  vertical  meridians  of  corresponding  retinal  points 
diverge  upwards  to  form  an  angle  of  about  2°.  A  slighter 
incongruence  may  also  occur  in  the  horizontal  meridian. 

Neglected  Diplopia. — It  is  clear  that  at  no  moment  in 
our  ordinary  life  can  stimulation  of  the  two  retinae  be 
limited  to  corresponding  points.  In  other  words,  the  single 
images  of  certain  objects,  arising  from  the  correspondence 
of  bi-retinal  points,  must  always  be  accompanied  by  the 
double  images  of  other  objects,  arising  from  the  disparation 


276  EXPERIMENTAL  PSYCHOLOGY 

of  other  points  simultaneously  excited.  But  we  neglect 
this  constant  diplopia,  just  as  we  neglect  the  blindness  of 
each  blind  spot  (exp.  116)  or  the  colour  blindness  of  the 
peripheral  retina  (exp.  52). 

The  Horopter. — The  "  horopter  "  is  a  line  or  surface  in 
the  field  of  vision,  whose  constituent  points  fall  on  corre- 
sponding retinal  points  in  a  given  position  of  the  eyes.  The 
forms  of  the  horopter  have  been  determined,  both  by 
mathematical  and  by  empirical  methods,  for  different 
positions  of  the  eye. 

The  Cyclopean  Eye. — Under  ordinary  circumstances  we 
localise  binocularly-seen  objects  in  a  direction  midway 
between  the  two  eyes  (exps.  126, 127),  and  in  uniocular  vision 
our  localisation  rests  usually  on  this  basis.  In  other  words, 
the  two  eyes,  whether  used  separately  or  combined,  tend 
to  function  in  regard  to  localisation  as  a  single  median 
"cyclopean"  eye.  This  tendency  has  to  be  suppressed  in 
shooting  and  in  other  uniocular  exercises. 

Depth  and  Disparation. — Provided  that  the  disparation 
is  relatively  slight,  the  simultaneous  stimulation  of  non- 
corresponding  points  on  the  two  retinse  still  permits  of 
single  vision.  The  degree  of  disparation  that  is  consistent 
with  the  absence  of  diplopia  varies  to  some  extent  with 
practice.  But  although  the  image  of  the  point  (or  object) 
is  not  doubled,  its  relative  position  in  space  becomes 
changed.  If  the  disparation  be  due  to  excessive  distance 
of  the  retinal  points  of  stimulation  from  one  another,  the 
object  appears  to  be  nearer,  and  if  it  be  due  to  insufficient 
distance,  the  object  appears  to  lie  farther,  than  the  position 
which  is  ascribed  to  the  single  image  obtained  from  truly 
corresponding  points. 

Fig.  9,  if  now  regarded  in  a  different  sense,  is  useful  in 
making  this  relation  clearer.  Let  X'  and  X"  represent  two 
objects,  the  rays  from  which  fall  on  the  now  moderately 
disparate  points  a^  and  x2.  So  long  as  the  disparation  is 
not  excessive,  binocular  combination  is  still  possible,  a  single 


BINOCULAR  EXPERIENCE  277 

object  is  seen  and  it  is  referred  to  X,  which  lies  nearer  than 
the  fixation  point  F.  On  the  other  hand,  if  0'  arid  0"  be  the 
two  objects,  and  if  the  disparation  is  due  to  insufficient 
distance  of  the  twin  retinal  points  ol  and  02,  binocular  com- 
bination is  again  possible,  provided  that  the  disparation  is 
not  too  great ;  but  the  single  object  is  referred  to  0,  a  point 
more  distant  than  the  fixation  point  (exp.  128). 

Thus  the  apparent  distance  of  objects  (farther  or  nearer 
than  the  fixation  point)  is  related  to  the  nature  and  degree 
of  retinal  disparation.  According  to  Hering,  retinal  dis- 
paration is  the  innate  physiological  datum  on  which  our 
experiences  of  depth,  and  of  distance  relative  to  the  fixa- 
tion point,  are  directly  based. 

Depth  and  Eye  Movements. — On  the  other  hand,  it  has 
been  urged  that  such  experiences  depend  not  directly  on 
retinal  disparation,  but  on  the  kinaesthetic  sensations  arising 
from  movements  of  each  eyeball  and  lens,  i.e.  from  changes 
in  binocular  fixation  and  accommodation,  which  are  pro- 
voked to  correct  the  diplopia  and  to  obtain  well-defined 
images  respectively.  How  far  these  muscular  sensations 
play  a  part  in  the  development  of  distance  perception  is 
uncertain ;  but  the  following  considerations  show  that  they 
are  by  no  means  essential,  so  far,  at  least,  as  concerns  adult 
experience. 

The  distances  of  falling  objects  from  the  fixation  point 
may  be  compared  when  they  are  visible  to  the  eyes  for 
so  short  a  time  as  to  preclude  orbital  movement  (exp.  129). 
The  characteristic  depth  of  stereoscopic  pictures  persists 
when  the  stereoscope  is  only  momentarily  illuminated — 
again  precluding  movement.  Stereoscopic  effects  may  be 
obtained  by  combining  the  after-images  of  stereoscopic 
pictures,  either  when  the  latter  have  been  regarded  simul- 
taneously or  even  after  they  have  been  regarded  suc- 
cessively. 

On  the  other  hand,  in  favour  of  the  influence  of  kin- 
sesthetic  sensations  upon  the  perception  of  distance,  the 


278  EXPERIMENTAL  PSYCHOLOGY 

following  experiment  has  been  adduced.  If  one  of  two 
vertical  black  threads,  both  seen  before  a  uniform  white 
ground  by  one  eye,  be  made  to  approach  the  observer,  he 
will  often  be  able  to  tell  whether  the  one  is  advancing  or  is 
at  a  different  distance  from  the  other,  although  the  size  of 
the  retinal  image  is  practically  unchanged.  This  ability  has 
been  attributed  to  sensations  arising  from  accommodation. 
It  is,  however,  difficult  completely  to  exclude  all  other 
factors  (difference  of  light  and  shade,  for  instance,  and  even 
minute  changes  in  size)  which  may  influence  our  perception 
of  distance.  Moreover,  in  a  similar  but  more  delicate 
experiment,  wherein  the  subject  uniocularly  regards  the 
border  line  that  vertically  divides  two  surfaces,  one  black, 
the  other  white,  from  one  another,  he  is  found  incapable  of 
judging  the  distance  of  the  movable  black  screen  from  him, 
or  of  telling  whether  it  is  being  moved  to  or  from  the 
stationary  white  background. 

It  is  certain  that  the  muscular  sensations  of  the  eyeballs 
afford  us  scant  knowledge  of  the  position  of  the  eyes  in 
darkness.  When  the  finger  is  held  up  in  the  dark  and 
attempts  are  made  to  turn  the  eyes  to  it,  considerable  in- 
accuracies in  convergence  are  found  to  occur.  Or  if  the 
eyes  be  fixed  on  a  bright  spot  in  an  otherwise  dark  room, 
and  fixation  be  as  accurately  as  possible  maintained  after 
extinction  of  that  light,  the  eyeballs  gradually  wander  with- 
out the  subject  being  aware  of  their  movement.  Careful 
observation  shows  that  the  accurate  preservation  of  fixation 
for  brief  periods  even  in  daylight  is  impossible ;  such  errors 
being,  of  course,  negligible  when  compared  with  those  which 
involuntarily  occur  in  the  absence  of  light.  These  con- 
siderations make  it  additionally  difficult  to  accept  the  view 
that  kinsesthetic  sensations  of  orbital  origin  are  of  funda- 
mental importance  in  adults  for  the  perception  of  the  third 
dimension. 

On  the  other  hand,  it  would  be  rash  to  assert  that  orbital 
and  ciliary  muscle  sensations  are,  always  and  wholly,  with- 


BINOCULAR  EXPERIENCE  279 

out  influence  in  this  direction.  They  are  conceivably  of 
considerable  importance  in  infancy,  when  they  may  aid  in 
the  elaboration  of  the  innate  system  of  corresponding  and 
disparate  points  and  of  retinal  local  signs,  and  thus  help  to 
complete  the  meaning  which  correspondence  and  disparation 
come  to  possess. 

It  is  noteworthy  that  squinters,  in  whom,  of  course,  the 
normal  congenital  relation  between  corresponding  points 
is  utterly  overthrown,  come  either  to  neglect  the  visual 
experiences  of  the  squinting  eye,  or  to  elaborate  a  totally 
new  system  of  relations  between  pairs  of  points  in  the  two 
retinse.  By  the  latter  procedure  they  acquire  single  vision 
by  binocular  combination,  although  they  never  altogether 
lose  the  congenital  system  of  interocular  relationship.  At 
present  we  are  ignorant  whether  or  not  kinsesthetic  sensa- 
tions play  any  part  in  the  construction  of  this  second 
system.  It  is  also  uncertain  whether  those  persons  who 
rely  solely  on  uniocular  vision  depend  for  their  appreciation 
of  depth  on  other  than  the  "  psychological "  factors  to  which 
we  have  now  to  refer. 

Depth  and  Psychological  Factors. — Distances  can  be  still 
discriminated,  even  when  the  objects  lie  so  far  away  (say, 
beyond  20  metres)  that  any  differences  due  to  retinal  dis- 
paration or  kinsesthesis  must  be  negligible.  We  are  then 
entirely  dependent  on  inference.  We  estimate  such  distances 
or  depths  by  differences  in  distinctness,  in  size,  or  in  light 
and  shade.  The  illusions  of  distance,  which  occur  in  excep- 
tionally clear  or  foggy  weather,  show  to  what  an  extent  we 
are  habitually  dependent  on  these  "psychological"  factors. 
Their  importance  is  further  instanced  by  the  observation 
that  the  stereoscopic  effects  of  a  painting  are  far  more 
striking  in  uniocular  than  in  binocular  vision.  The  two 
eyes,  with  their  usual  accuracy,  insist  that  the  canvas  is 
flat;  the  effects  of  size,  colour,  light  and  shade  are  more 
effective  in  suggesting  relief  when  the  painting  is  viewed  by 
a  single  eye. 


280  EXPERIMENTAL  PSYCHOLOGY 

Binocular  Rivalry.  —  When  corresponding  points  are 
stimulated  by  unlike  stimuli,  either  combination  or  rivalry 
results.  Coloured  squares  of  equal  size  and  brightness  may 
be  combined  without  difficulty.  The  more  the  two  presenta- 
tions differ  in  contour  or  in  brightness,  or  the  more  they  are 
incongruous  in  general  meaning,  the  more  impossible  becomes 
binocular  combination.  Instead  of  combining,  the  two 
impressions  alternate,  sometimes  one,  sometimes  the  other, 
occupying  the  field  of  consciousness.  The  field  can  be  to  a 
great  extent  limited  to  one  or  other  eye  by  the  control  of 
attention,  or  by  the  relative  intensity  or  insistence  of  the 
sensations,  either  retinal  or  muscular,  derived  from  the  two 
eyes.  One  of  the  images  can  be  completely  suppressed  by 
practice  (exp.  130). 

[The  effect  of  combining  black  and  white  fields  binocularly 
is  to  produce  a  varying  shade  of  grey  which  has  a  metallic 
lustre  (exp.  130).  The  lustre  of  rough  surfaces  in  uniocular 
vision  is  probably  due  to  similar  differences  in  the  intensity 
of  stimulation  of  neighbouring  retinal  points,  owing  to  the 
irregular  reflection  of  light  from  such  surfaces.  Thus  the 
phenomenon  would  be  an  instance  of  a  pair  of  neighbouring 
points  on  the  single  retina  being  able  to  evoke  the  same 
experience  as  a  pair  of  slightly  disparate  (or  corresponding) 
points  on  the  two  retinae.] 

[Uniocular  Eelief. — If  this  were  so,  we  might  expect 
uniocular  stereoscopy  to  result  when  the  image  of  an  object 
is  so  thrown  on  the  single  retina  as  to  stimulate  in  rapid 
alternation  (i.)  given  points  (a^  bv  cl  .  .  .)  and  (ii.)  neigh- 
bouring points  (a2,  b2,  c2  .  .  .)  of  the  same  retina  corresponding 
to  points  on  the  opposite  retina  which  are  slightly  disparate 
to  the  points  av  bv  c1.  .  .  .  We  have  experimental  evidence 
that  under  such  conditions  uniocular  relief  does  actually 
occur.  Its  occurrence  promises  to  throw  much  light  on  the 
physiological  and  anatomical  basis  of  our  perception  of 
depth.  It  may  form  an  important  factor  in  the  spatial 
perception  of  one-eyed  individuals. 


BINOCULAR  EXPERIENCE  281 

A  very  different  form  of  uniocular  relief  occurs  when 
blue  and  red  letters  are  uniocularly  regarded  on  a  black 
ground.  The  letters  of  one  colour  appear  to  be  nearer  or 
farther  than  those  of  the  other  colour,  according  to  the 
individual  and  the  direction  from  which  he  regards  them. 
Such  colour  relief  has  been  attributed  to  individual  differ- 
ences in  the  eccentric  position  of  the  pupil  with  respect  to 
the  visual  axis,  and  in  the  position  of  the  coloured  "  circles 
of  diffusion  "  on  the  retina  which  differ  according  as  the  eye 
is  accommodated  for  one  colour  or  for  the  other.  The  borders 
of  the  letters  appear  light  or  dark,  and  these  apparent 
differences  of  shading,  varying  with  the  nasal  or  temporal 
eccentricity  of  the  pupil  and  with  the  direction  of  regard, 
suggest  relief.] 

Binocular  Contrast. — Under  certain  conditions,  a  coloured 
stimulus  applied  to  one  eye  can  be  made  to  evoke  the  con- 
trast colour  sensation  in  the  opposite  eye  (exps.  131,  132, 
133).  The  close  resemblance  of  binocular  to  uniocular 
contrast  compels  us  to  suppose  either  that  colour  sensations 
are  of  more  central  origin  than  is  commonly  believed,  or 
that  a  direct  nervous  connection  exists  between  the  two 
retinae  whereby  stimulation  of  one  eye  leads  to  retinal 
changes  in  the  opposite  eye.  Such  nervous  connection  has 
been  definitely  demonstrated.  Further,  we  know  that 
movement  of  the  cones,  produced  in  one  eye,  leads  to 
cone  movement  in  the  opposite  eye.  But  this  reaction 
is  too  slow  to  explain  the  immediacy  of  binocular  con- 
trast. It  has  been  suggested  that  the  electrical  variation 
in  the  two  eyes,  which  is  known  to  occur  after  uniocular 
stimulation,  may  be  the  physiological  basis  of  binocular 
contrast. 

[Binocular  Flicker  and  Brightness. — When  two  flickering 
discs  of  light,  precisely  similar  in  all  respects,  are  thrown 
on  corresponding  areas  of  the  two  retinae,  and  the  two 
retinal  images  are  binocularly  combined,  it  is  found  that  the 
rate  of  intermittence  necessary  to  extinguish  flicker  in  the 


282  EXPERIMENTAL  PSYCHOLOGY 

binocular  image  is  almost  exactly  the  same  as  the  rate 
necessary  to  extinguish  flicker,  when  only  one  of  the  discs 
is  seen,  uniocularly. 

When  two  such,  binocularly  combined  flickering  discs, 
are  arranged  so  that  the  component  dark  and  bright 
phases  of  each  flicker  synchronise  in  the  two  eyes,  and  are 
compared  with  two  other  flickering  discs,  similarly  combined 
but  arranged  so  that  the  phases  alternate, — the  bright 
phases  of  the  flicker  in  one  eye  being  synchronous  with  the 
dark  phases  in  the  other, — the  rate  of  rotation  at  which 
flicker  is  extinguished  is  approximately  the  same  for  the 
two  images  arising  by  combination  of  the  two  pairs.  There 
appears  to  be  practically  no  interference  or  combination 
between  corresponding  points  of  the  two  retinae  under  these 
conditions,  however  different  be  the  phases  of  reaction  of 
these  points  at  any  moment.  Moreover,  when  flicker  is 
extinguished,  there  is  not  the  slightest  difference  in  bright- 
ness between  the  combined  images.  That  is  to  say,  no 
trace  exists  of  a  Talbot- Plateau  law  (page  86)  applicable  to 
an  imaginary  single  retina,  the  functional  composite  of  the 
two  retinae. 

On  the  other  hand,  when  a  steady  flickerless  image  is 
received  on  one  retina,  and  a  flickering  image  is  simultane- 
ously received  on  the  corresponding  area  of  the  other  retina, 
the  flickering  of  the  latter  is  very  distinctly  damped,  and  the 
extent  of  damping  seems  to  remain  constant  throughout  a 
considerable  range  of  variation  in  the  brightness  of  the 
flickerless  image. 

It  would  therefore  appear  that,  while  there  is  practically 
no  binocular  interaction  between  two  flickering  images 
binocularly  combined,  there  is  a  marked  interaction  when 
one  of  the  eyes  receives  a  steady,  in  place  of  a  flickering, 
light  stimulus.  This  effect  of  a  steady  upon  a  flickering 
image  appears  comparable  to  the  interaction  of  two  uniocular 
steady  images,  to  which  we  shall  refer  immediately.  But 
we  have  yet  to  explain  why  a  steady  image  is  able,  while  a 


BINOCULAR  EXPERIENCE  283 

flickering  image  is  unable,  to  affect  the  image  of  the  corre- 
sponding area  of  the  other  eye. 

Ifc  might  be  thought  that  a  certain  stage  in  the  elabora- 
tion of  the  results  of  retinal  stimulation  must  be  reached 
before  such  stimulation  can  influence  experiences  derived 
from  the  other  eye.  Yet,  from  the  psychological  standpoint, 
a  flickering  light  is  as  elaborate  a  conscious  state  as  a  steady 
light.  Moreover,  on  the  physiological  side,  it  is  difficult  to 
conceive  a  mechanism  which  shall  at  once  account  for  the 
facts  of  retinal  correspondence,  cyclopean  vision,  cortical 
hemianopia  and  conjugate  orbital  movements,  on  the  one 
hand,  and  for  the  recently  discovered  facts  that  we  have 
just  been  describing  on  the  other.  Certainly  the  simple 
diagrams  of  the  bi-retinal  relation  which  have  hitherto 
contented  us  are  now  quite  inadequate.  They  may  account 
for  all  motor  reflexes,  but  they  cannot  account  for  all 
binocular  conscious  processes.] 

The  brightness  of  the  combined  image  of  two  steady 
flickerless  lights,  respectively  thrown  on  corresponding 
retinal  areas,  is  obviously  not  equal  to  the  sum  of  the  two 
brightnesses.  Its  value  is  usually  slightly  above  the  arith- 
metical mean  of  the  brightness  of  the  uniocular  components, 
provided  that  these  do  not  differ  too  widely  from  one 
another.  When  the  components  are  equally  bright,  the 
binocular  increase  has  been  variously  estimated  to  be  from 
one-tenth  to  one-thirtieth.  It  appears,  however,  that  the 
increment  rises  with  the  degree  of  dark  adaptation. 

Fcclmer's  Paradox. — When  the  uniocular  images  differ 
more  widely  in  brightness,  the  binocular  brightness  becomes 
considerably  less  than  the  brighter  of  the  uniocular  com- 
ponents. This  result  is  known  as  "Fechner's  Paradox" 
(exp.  134).  It  reaches  its  maximum  when  the  two  bright- 
nesses are  in  the  ratio  1 :  25.  When  they  are  in  the  ratio 
1 : 1'5,  the  binocular  brightness  is  about  equal  to  that  of  the 
brighter  component.  Even  if  unequal  variations  in  the  size 
of  the  pupils  be  eliminated,  either  by  the  use  of  small 


284  EXPERIMENTAL  PSYCHOLOGY 

artificial  pupils  or  by  paralysing  the  pupils  by  means  of 
atropin,  Fechner's  paradox  persists.  It  is  absent  when  the 
darker  of  the  two  fields  is  presented  before  the  brighter,  and 
when  the  area  of  the  former  is  made  considerably  smaller 
than  that  of  the  latter.  No  satisfactory  explanation  of 
Fechuer's  paradox  has  been  yet  advanced.  Believing  that 
the  cortical  paths  of  corresponding  elements  of  the  two 
retinae  are  not  identical,  McDougall  has  suggested  that  the 
paradox  may  be  due  to  the  competition  of  the  cerebro-retinal 
elements  of  each  eye  for  the  maximal  total  nervous  energy 
which  is,  at  the  moment,  available.  Owing  to  such  com- 
petition, the  brightness  contributed  by  the  cerebro-retinal 
elements  of  each  eye  is  less  when  the  two  eyes  are  stimulated 
simultaneously  than  when  they  are  stimulated  successively. 
Further,  what  each  eye  loses  on  this  lower  physiological 
level  does  not  reappear  in  the  higher  psychological  synthesis 
of  the  two  images. 

The  Study  of  Orbital  Movements. — The  movements  of  the 
eyes  have  been  studied  by  various  methods,  e.g.  by  observ- 
ing the  movements  of  a  vessel  of  the  conjunctiva,  of  a  mark 
on  the  eyeball,  or  of  after-images.  They  have  also  been 
studied  by  the  cinematograph. 

Listing's  law  states  that  the  axis  round  which  oblique 
movements  of  the  eyes  take  place,  is  in  the  same  plane  as 
the  axes  round  which  simple  vertical  and  horizontal  move- 
ments take  place  (exp.  135).  The  law  hence  denies  the 
occurrence  of  swivel  (or  wheel)  rotation  of  the  eyes  around 
the  optic  axis ;  but  it  is  found  only  to  hold  for  parallel,  or 
approximately  parallel,  positions  of  the  visual  axes.  Under 
certain  conditions,  e.g.  when  the  eyes  converge  and  move 
obliquely,  or  when  the  head  is  inclined  laterally,  swivel 
rotation  undoubtedly  occurs. 

Under  ordinary  circumstances,  a  very  close  association 
exists  between  binocular  movement  and  accommodation,  the 
lens  becoming  more  convex  with  convergence,  and  less 
convex  with  divergence.  But  practice  under  certain  con- 


BINOCULAR  EXPERIENCE  285 

ditions  (for  example,  the  habitual  wearing  of  prismatic 
glasses)  can  overcome  this  association  and  supplant  it  by 
one  of  different  kind.  Convergence  and  increased  accom- 
modation are  also  associated  with  contraction  of  the  pupil. 


BIBLIOGRAPHY. 

"W.  Wundt,  Beitrdge  zur  Theorie  d.  Sinneswahrnehmung,  Leipzig  u. 
Heidelberg,  1862,  105;  Grundzilge  d.  physiol.  Psychol.,  Leipzig,  1902,  ii. 
587.  E.  Hering,  "Der  Raumsinn  u.  d.  Bewegungen  d.  Augen,"  in 
Hermann's  Handbuch  d.  PhysioL,  1879,  iii.  Ite  Theil,  343;  Beitrdge  zur 
PhysioL,  Leipzig,  1861-4,  i.-v.  H.  L.  F.  von  Helmholtz,  Handbuch  d. 
physiol.  OptiJc,  2te  Aufl.,  Hamburg  u.  Leipzig,  1896,  841.  F.  Hillebrand, 
"Das  Verhaltniss  von  Accomodation  u.  Konvergenz  zur  Tiefenlokalisa- 
tion,  Ztsch.f.  Psychol.  u.  PhysioL  d.  Sinnesorgane,  1894,  vii.  97.  E.  T. 
Dixon,  "On  the  Relation  of  Accommodation  and  Convergence  to  our  Sense  of 
Depth,"  Mmd,  N.S.,  1895,  iv.  195.  W.  H.  R.  Rivers,  "  Vision  "  in  Schafer's 
Text-Book  of  Physiology,  Edinburgh  and  London,  1900,  ii.  1099,  1122. 
E.  B.  Titchener,  Experimental  Psychology,  New  York,  1901,  ii.  pts.  1,  2, 
chap.  ix.  F.  B.  Hofrcann,  "Die  neueren  Untersachungen  iiber  d.  Sehen  d. 
Schielenden,"  in  Ergebnisse  d.  PhysioL,  1902,  2te  Abth.  801.  J.  W.  Baird, 
"The  Influence  of  Accommodation  and  Convergence  upon  the  Perception  of 
Depth,"  Amer.  Jour,  of  Psychol.,  1903,  xiv.  150.  C.  S.  Sherrington,  "On 
Binocular  Flicker  and  the  Correlation  of  Activity  of  'Corresponding'  Retinal 
Points,"  Brit.  Jour,  of  Psychol.,  1904-5,  i.  26.  W.  M'Dougall,  "Note  on 
the  Principle  underlying  Fechner's  'Paradoxical  Experiment,'"  etc.,  ibid.  114. 
M.  Straub,  "  Ueber  monokulare  korpeiiiche  Sehen,"  u.s.w.,  Ztschr.f.  Psychol. 
u.  PhysioL  d.  Sinnesorgane,  1904,  xxxvi.  431.  0.  Zoth,  "Augenbewegungen 
u.  Gesichtswahrnehmungen,"  in  Nagel's  Handbuch  d.  PhysioL  d.  Menschen, 
Braunschweig,  1905,  iii.  335. 


CHAPTEE    XXI 
ON   BINAURAL  EXPERIENCE 

THE  DIRECTION  OF  SOUNDS. 

Binaural  Differences  and  the  Perception  of  Sound. — The 
inequality  in  effect  of  a  given  sound  upon  the  two  ears 
plays  an  important  part  in  determining  our  appreciation 
of  its  direction.  It  is  a  familiar  experience  that,  when  the 
source  of  a  sound  is  placed  asymmetrically  with  regard  to 
the  two  ears,  it  can  be  localised  with  tolerable  accuracy. 
With  eyes  closed  and  with  head  unmoved,  we  can  correctly 
judge  whether  the  source  of  the  sound  lies  to  our  right  or 
to  the  left,  and,  with  less  precision,  whether  it  lies  towards 
our  front  or  back.  When,  on  the  other  hand,  the  source 
of  sound  is  placed  symmetrically  with  regard  to  the  two 
ears, — when,  that  is,  the  sound  lies  behind,  above,  or  below 
the  head,  in  the  sagittal  plane,1 — our  estimation  of  its 
direction  is  notoriously  very  erratic. 

Tactual  Differences. — The  binaural  differences,  in  virtue 
of  which  we  are  able  to  localise  a  given  sound,  have  been 
referred  by  Wundt  and  others  to  the  tactual  sensations 
evoked  by  sound  vibrations  from  the  two  auricles  and 
tympanic  membranes.  But  the  evidence  derived  from 
introspection  and  from  the  study  of  subjects  who  are  devoid 
of  auricle  and  drum,  lends  little  support  to  this  view. 

Labyrinthine  Differences. — The  suggestion  has  also  been 

1  The  sagittal  plane  is  the  plane  of  the  sagittal  suture  of  the  skull,  the 
median  plane  of  the  body. 


BINAURAL  EXPERIENCE  287 

put  forward  by  Preyer  that  our  powers  of  auditory  localisa- 
tion are  connected  with  the  peculiar  orientation  of  the  three 
pairs  of  semicircular  canals  in  the  three  planes  of  space, 
the  various  canals  being  differently  stimulated  according  to 
the  direction  of  the  sound.  Miinsterberg  supposes  that  the 
effect  of  auditory  stimulation  of  a  given  canal  would  be  to 
cause  the  head  reflexly  to  turn  in  the  plane  of  that  canal, 
and  that  the  kinsesthetic  sensations  arising  from  such 
various  head  movements  form  the  basis  of  our  judgment 
of  sound  direction.  But  apart  from  other  objections,  it  is 
inconceivable  that  sounds,  coming  from  different  directions, 
should  differently  stimulate  the  semicircular  canals ;  for 
by  the  time  they  reach  the  inner  ear,  surely  all  vibrations 
of  sound  must  have  the  same  direction,  however  different 
their  place  of  origin. 

Temporal  Differences. — A  third  possible  cause  of  the 
unlike  effects  produced  by  a  sound  on  the  two  ears  lies 
in  temporal  differences  of  stimulation.  It  is  obvious  that 
a  sound,  the  source  of  which  is  on  one  side  of  the  head, 
must  affect  one  ear  before  the  other.  But  we  have  no 
evidence  that  such  successiveness  of  excitation  is  essential 
for  sound  localisation.  On  the  contrary,  we  are  able 
uninterruptedly  to  localise  continuous  sounds.  And  if  two 
forks  of  identical  pitch  be  held  to  opposite  ears  and  be 
continuously  sounded,  we  hear  a  single  sound  localised 
either  in  the  middle  line  or  to  one  side  of  it,  according 
as  the  two  forks  are  sounding  with  equal  or  unequal 
intensity. 

Phase  Differences. — There  are  but  three  other  conceivable 
ways  in  which  a  given  source  of  sound  may  differently  affect 
the  two  ears,  namely,  by  virtue  of  differences  in  phase,  in 
timbre,  or  in  intensity.  By  differences  in  timbre  we  mean 
differences  in  the  relative  intensity  of  the  overtones  to  one 
another  and  to  the  fundamental  tone.  By  difference  of 
phase  we  mean  the  effect  due  to  the  unequal  distances  of 
the  two  ears  from  the  single  source  of  sound,  in  consequence 


288  EXPERIMENTAL  PSYCHOLOGY 

of  which  the  sound  waves,  falling  at  any  moment  on  the 
two  ears,  are  in  unlike  phase.  We  can  experimentally 
show  that  changes  in  the  phase  relation  of  the  sound 
stimuli  reaching  the  two  ears  produce  distinct  changes  in 
the  apparent  direction  of  the  sound,  whether  the  source  of 
sound  for  each  ear  is  the  same  or  different  (exps.  138,  139). 
Owing,  however,  to  the  easy  conduction  of  sounds  through 
the  head  from  ear  to  ear  (page  21),  these  changes  of 
localisation,  which  at  first  sight  appear  to  be  directly  due 
to  dissimilarity  of  phase,  may  really  be  due  to  differences 
of  intensity  in  the  two  ears;  the  differences  of  intensity 
arising  from  the  interaction  (by  interference  or  summation) 
of  the  two  series  of  waves  of  different  phase. 

According  to  Myers  and  Wilson,  binaural  phase  differ- 
ences owe  their  effect  to  the  binaural  intensity  differences 
which  they  thus  produce ;  hence  it  becomes  unnecessary  to 
adopt  Lord  Kayleigh's  view  that  we  are  able  to  tell  at  which 
ear  the  phase  of  vibration  is  in  advance,  when  we  judge  the 
direction  of  a  laterally  placed  sound  of  low  pitch.  This 
view  is  inevitable,  so  long  as  we  suppose  that  such  a  sound 
can  only  reach  the  more  distant  ear  by  the  air.  For  it  is  a 
fact  that  the  head  is  not  large  enough  to  throw  an  appreci- 
able sound  shadow  when  the  sound  is  of  low  pitch  and 
consequently  of  very  great  wave  length.  But  the  difficulty 
at  once  disappears,  if  we  bear  in  mind  how  readily  a  sound 
is  transmissible  from  ear  to  ear  by  bone  conduction.  Then 
it  is  easily  demonstrable  that  the  phase  differences  at  the 
two  ears  produce  differences  of  intensity. 

Intensity  Differences. — Indeed,  there  is  ample  evidence 
that  binaural  differences  of  intensity  play  the  essential  part 
in  sound  localisation.  Other  things  being  equal,  the  sound 
is  localised  on  that  side  which  receives  the  stronger  stimulus 
(exps.  136,  137).  The  importance  of  binaural  differences 
of  intensity  is  well  illustrated  by  the  two  following  facts, 
chosen  from  many  others. 

In  the  first  place,  a   tuning-fork   placed  anywhere  on 


BINAURAL  EXPERIENCE  289 

the  head  is  localised  in  that  ear  which  receives  the  stronger 
stimulus. 

Secondly,  let  an  imaginary  circle,  a  metre  or  more  in 
diameter,  be  drawn  horizontally  round  the  head  at  the  level 
of  the  ears,  its  centre  lying  midway  between  them  ;  and  let 
this  circle  be  divided  into  quadrants,  limited  by  the  sagittal 
and  the  coronal  planes.1  Then  it  is  found  that  a  sound, 
placed  at  any  point  on  the  circumference  of  this  circle  (save 
exactly  midway  before  or  behind  the  head),  is  accurately 
judged  by  the  subject  to  lie  to  his  right  or  his  left.  But  he 
is  apt  to  confuse  the  anterior  with  the  posterior  quadrants 
of  the  same  side.  That  is  to  say,  he  often  wrongly  supposes 
that  a  sound  is  as  much  in  front  of  the  transverse  plane  as 
it  is  really  behind  that  plane  (exp.  137).  These  confusions 
are  just  what  one  would  expect  on  the  basis  of  binaural 
differences  of  sound  intensity. 

Timbre  Differences. — Inasmuch  as  the  shadow  effect  of 
an  obstacle  increases  with  the  shortness  of  the  wave  length 
of  the  sound,  it  follows  that  the  sound  shadow  cast  by 
the  head  must  be  different  for  the  different  overtones  that 
are  present  in  a  given  sound.  That  is  to  say,  a  laterally 
placed  sound,  reaching  one  side  of  the  head,  will  be  of 
different  timbre  from  that  reaching  the  other  side  through 
the  air. 

There  is,  however,  another  way  in  which  the  apparent 
timbre  of  a  sound  may  alter  with  change  in  the  position  of 
the  sound.  We  have  evidence  that  the  auricle  itself  has 
considerable  influence  in  modifying  the  timbre  (and  intensity) 
of  sounds,  according  to  the  direction  in  which  they  travel  to 
the  ear.  The  ticks  of  a  watch,  placed  in  front  of  the  head, 
are  of  very  different  quality,  compared  with  those  of  the 
same  watch,  placed  behind  the  head.  And  when  artificial 
auricles  or  flaps  are  attached  to  the  ears,  capable  of  being 
turned  in  any  desired  direction,  various  illusions  of  localisa- 

1  The  coronal  plane  is  a  vertical  plane  between  the  two  ears  perpendicular 
to  the  sagittal  plane. 

19 


290  EXPERIMENTAL  PSYCHOLOGY 

tion  can  be  produced,  according  as  one  or  other  ear  is  more 
favourably  or  unfavourably  exposed  to  the  partial  tones  of 
the  sound.  It  has  been  urged  by  Mach  that  the  auricles 
act  as  resonators,  their  effect  depending  on  their  position 
relatively  to  the  direction  of  the  impinging  sound  waves. 

No  attempt  has  yet  been  made  to  investigate  the  effects 
on  our  judgment  of  direction,  experimentally  produced  by 
conducting  to  separate  ears  two  tones  of  like  pitch  but  of 
unlike  timbre.  Nevertheless  the  importance  of  the  timbre 
is  well  shown  in  the  extraordinary  improvement  effected  by 
practice  in  localising  sounds  which  are  in  the  sagittal  plane 
in  front  of,  behind,  above,  or  below  the  head.  When  sounds, 
thus  placed,  are  consecutively  given,  their  correct  localisation 
must  surely  depend  upon  successive  differences  in  their 
timbre. 

If  differences  of  timbre  be  so  important,  localisation 
should  be  possible  when  only  one  ear  is  stimulated.  But 
owing  to  the  ease  with  which  sounds  travel  by  bone  con- 
duction from  one  ear  to  the  other,  it  is  only  possible  to 
investigate  the  effects  of  uniaural  stimulation  in  the  case 
of  individuals  who  are  known  to  be  absolutely  deaf  in  one 
ear.  In  such  subjects,  localisation  is  extremely  defective 
in  regard  to  sounds  which  are  placed  on  their  deaf  side. 
Yet  here,  again,  considerable  improvement  occurs  with 
practice. 

Now  these  improvements  of  localisation  that  are  produced 
by  practice,  alike  in  the  binaural  hearing  of  sounds  in  the 
middle  line  (page  286)  and  in  the  uniaural  hearing  of  sounds 
situated  on  the  deaf  side,  are  far  better  marked  for  noises, 
and  for  sounds  rich  in  overtones,  than  for  pure  tones.  The 
tones  of  tuning-forks,  for  example,  continue  to  be  localised 
with  great  inaccuracy  in  such  cases,  even  after  considerable 
practice.  Moreover,  the  subjects  introspectively  ascribe 
their  improvement  to  their  more  delicate  appreciation  of 
differences  of  timbre  of  the  sound  according  to  the  direction. 

It  may  seem   strange  that   even  the  most  unmusical 


BINAURAL  EXPERIENCE  291 

should  be  so  sensitive  to  differences  in  the  timbre  of  sounds 
as  to  be  able  to  localise  them  correctly.  Yet  this  is  hardly 
more  surprising  than  the  correspondingly  universal  ability 
to  identify  individuals  by  their  voice. 

Adaptation  in  Localisation. — Those  who  are  partially 
deaf  in  one  ear  are  able,  nevertheless,  to  localise  sounds 
with  fairly  normal  accuracy.  When  the  cause  of  such 
partial  deafness  is  removable, — when,  for  example,  it  is  due 
to  the  accumulation  of  wax, — the  restoration  of  hearing  is 
accompanied  by  a  temporary  erroneous  localisation  of  sounds 
towards  the  recovered  side. 

When  prismatic  glasses  or  lenses  are  continuously  worn, 
so  that  the  visual  field  is  shifted  in  one  or  other  direction 
or  is  completely  inverted,  the  old  association  between  visual 
and  auditory  localisation  at  length  breaks  down,  and  sounds 
appear  to  come  from  the  new  visual  direction,  especially,  of 
course,  if  their  source  be  at  the  same  time  seen. 


THE  DISTANCE  OF  SOUNDS. 

Our  estimation  of  the  distance  of  sounds,  although  it  is 
entirely  independent  of  binaural  hearing,  may  be  considered 
here.  It  is  based  upon  the  intensity  and  the  timbre  of 
sounds,  and  upon  previous  experience. 

The  higher  overtones  relatively  predominate  in  distant 
sounds,  the  lower  in  nearer  sounds.  Thus  distant  sounds 
appear  sharper  than  nearer  ones  (page  31).  By  increasing 
the  pressure  within  the  middle  ear,  the  free  vibration  of  the 
ossicles,  which  subserve  the  transmission  of  lower  tones,  is 
impeded,  while  the  higher  tones  can  in  large  measure  pass 
by  direct  bone  conduction  (page  20).  This  condition  thus 
gives  rise  to  the  illusion  that  near  voices  are  at  a  great 
distance  from  the  ear. 

In  general,  however,  distance  produces  a  greater  effect  on 
high  than  on  low  tones.  The  effect  is  also  greater  on  noises 
than  on  tones  and  on  consonants  than  on  vowels.  The 


292  EXPERIMENTAL  PSYCHOLOGY 

predominance  of  the  lower  overtones  in  near  sounds,  to 
which  we  have  just  alluded,  may  be  due  to  the  obliterating 
power  possessed  by  low  tones  (exp.  26). 


BIBLIOGRAPHY. 

E.  Mach,  "  Bemerkungen  iiber  d.  Function  d.  Ohrmuschels, "  Arch.f. 
Ohrenheilkunde,  1875,  ix.  72.  Lord  Rayleigh,  "Our  Perception  of  the 
Direction  of  a  Source  of  Sound,"  abstracted  in  Nature,  1876,  xiv.  32  ; 
Philosoph.  Mag.,  1907  (6)  xiii.  214  ;  "Acoustical  Observations,"  ibid.,  1877 
(5)  iii.  456.  S.  P.  Thompson,  ' '  On  Binaural  Audition, "  Philosoph.  Hag. ,  1877 
(5)  iv.  274  ;  ibid.,  1878  (6)  vi.  383;  ibid.,  1881  (5)  xii.  351.  C.  Stumpf, 
Tenpyychologie,  Leipzig,  1883,  i.  208,  242,  395.  W.  Preyer,  "Die  Wahr- 
nehmung  d.  Schallrichtuug  mittelst  d.  Bogengiinge,"  Arch.  f.  d.  ges. 
PhysioL,  1887,  xl.  586.  H.  Miinsterberg,  "Der  Raumsinn  d.  Qhres,"  Beitr. 
zur  experim.  PsychoL,  1889,  ii.  182.  M.  Matsumoto,  "  Researches  in  Acoustic 
Space,"  Studies  from  the  Yale  PsychoL  Lab.,  1897,  v.  1.  A.  H.  Pierce, 
Studies  in  Auditory  and  Visual  Space  Perception,  New  York,  1901.  J.  R. 
Angell  and  W.  Fite,  "The  Monaural  Localisation  of  Sound,"  PsychoL  Rev., 
1901,  viii.  225,  449.  W.  4Wundt,  Grundzuge  d.  physiol.  PsychoL  5te 
Aufl.,  Leipzig,  1902,  ii.  486.  J.  R.  Angell,  "A  Preliminary  Study  of  the 
Significance  of  Partial  Tones  in  the  Localisation  of  Sounds,"  PsychoL  Rev., 
1903,  x.  1.  C.  S.  Myers  and  H.  A.  Wilson,  "On  the  Perception  of  the 
Direction  of  Sound,"  Proc.  Roy.  Soc.,  1908,  A,  Ixxx.  260;  "The  Influence 
of  Binaural  Phase  Differences  on  the  Localisation  of  Sounds,"  Brit.  Journ. 
of  PsychoL ,  1908,  ii.  363. 


CHAPTER    XXII 


ON  THE  VISUAL   PERCEPTION   OF  SIZE  AND 
DIRECTION 

THE  visual  perception  of  size  depends  on  two  principal 
factors, — the  size  of  the  retinal  image  and  the  distance  at 
which  the  object  is  estimated  to  be. 

Micropsia  at  the  Fixation  Point. — The  size  of  the  retinal 
image  is  considerably  influenced  by  irradiation.  Irradiation 
occurs  in  consequence  of 
the  diffusion  of  light  on  to 
neighbouring  retinal  areas. 
Such  an  overflow  is,  as  we 
should  expect,  much  more 
marked  with  a  large  than 
with  a  small  pupil.  Ow- 
ing to  irradiation,  a  white 
square  on  a  black  ground 
appears  larger  than  an 
actually  equal  black  square 
on  a  white  ground.  The 
apparent  acuteness  of  the 
right  angles  within  the 
annexed  diagram  (fig.  10)  is  due  to  the  same  cause. 

When  the  pupil  is  dilated  under  the  influence  of  atropin, 
the  size  of  letters  printed  in  black  type  is  considerably 
reduced.  This  form  of  micropsia  under  atropin  is  likewise 
due  to  irradiation.  It  has  been  called  "  micropsia  at  the 
fixation  point." 


FIG.  10. 


293 


294  EXPERIMENTAL  PSYCHOLOGY 

The  Dependence  of  Apparent  Size  on  the  Relation  of  an 
Object  to  the  Fixation  Point. — The  apparent  size  of  an  object 
also  depends  on  its  position  relatively  to  that  of  the  fixation 
point.  We  have  already  seen  reasons  (page  277)  for  con- 
sidering the  fixation  point  as  the  central  point  or  nucleus  of 
our  binocular  field  of  vision,  in  terms  of  which  all  other, 
nearer  or  farther,  objects  in  the  field  are  interpreted. 

In  uniocular  vision  it  is  easy  to  convince  oneself  that 
objects  which  are  nearer  than  the  fixation  point  appear  to 
be  larger,  and  that  those  which  are  farther  than  the  fixation 
point  appear  to  be  smaller,  than  they  would  appear  when 
directly  fixated.  For  if  one  eye  be  closed  and  the  other  be 
fixed  on  a  near  point,  stationary  objects  beyond  the  fixation 
point,  e.g.  printed  letters,  will  seem  to  grow  smaller  or  larger, 
as  the  fixated  point  is  moved  nearer  to  or  farther  from  the 
eye.  These  changes  in  size  are  frequently  accompanied  by 
inferred  changes  in  position  of  the  stationary  objects.  The 
latter  may  appear  not  only  smaller  but  more  distant,  or  not 
only  larger  but  nearer  than  before.  Further,  the  changes 
occur  even  when  the  moving  point  of  fixation  has  only  an 
imaginary  existence. 

Mieropsia  beyond  the  Fixation  Point.  —  There  is  a 
micropsia,  occurring  under  the  influence  of  atropin,  which 
cannot  be  attributed  to  irradiation.  It  has  been  called 
"micropsia  beyond  the  fixation  point."  If  one  eye  be 
closed  and  if  the  other  atropinised  eye  make  an  effort  of 
accommodation  in  regarding  printed  type,  the  print  becomes 
markedly  reduced  in  size.  The  micropsia  also  occurs  when 
both  eyes  are  used,  provided  that  the  ciliary  muscle  of  each 
has  been  atropinised.  It  occurs,  indeed,  when  each  ciliary 
muscle  is  completely  paralysed. 

Many  observers  have  attributed  both  forms  of  the 
micropsia  produced  by  atropin — as  well  as  the  micropsia, 
which  may  occur  late  in  lif&  (in  presbyopia) — to  paresis  or 
weakness  of  the  ciliary  muscle.  In  consequence  of  this 
paresis,  it  is  supposed  that  a  greater  strain  is  thrown  on  the 


SIZE  AND  DIRECTION  295 

muscle;  that  intenser  kinsesthetic  sensations  arise  during 
attempted  accommodation ;  that  the  object  is  consequently 
assumed  to  be  nearer  than  it  really  is,  and  therefore  is  judged 
smaller.  We  have  pointed  out,  however,  that  one  form  of 
micropsia  under  atropin  is  due  to  irradiation  (page  293).  In 
the  other  form,  which  occurs  even  in  complete  paralysis  of 
accommodation,  it  is  difficult  to  understand  how  kinsesthetic 
sensations,  whether  of  ciliary  or  of  other  origin,  can  play 
any  part.  Under  these  circumstances,  we  are  compelled  to 
adopt  the  view  suggested  by  Eivers,  that  an  intended  but 
unaccomplished  change  of  fixation  point  can  produce  the 
same  effect  upon  the  apparent  size  of  an  object  as  would 
occur  if  the  volitional  impulse  had  been  able  to 
achieve  its  results. 

Influence  of  Estimated  Distance  on  Size. — The 
influence  of  the  estimated  distance  of  an  object 
on  its  apparent  size  is  shown  in  the  variable  size 


of   an  after-image  according  to  the  distance  of      FIG.  11. 
the   fixation   point    at   which    it    is    projected 
(exp.   140).      The  apparent    size    of    objects   is   similarly 
affected  by  a  foggy  or  unusually  clear  atmosphere,  or  by 
suggestions  of  perspective  in  a  drawing. 

Micropsia  in  Retinitis. — The  micropsia,  often  occurring 
in  certain  stages  of  retinitis,  is  doubtless  to  be  ascribed  to  a 
separation  of  the  retinal  end  organs  by  inflammatory  exuda- 
tion. The  local  signature  of  the  retinal  elements  persists 
unaltered,  and  thus  objects  appear  to  be  smaller  than  usual. 

Comparison  of  Horizontal  and  Vertical  Lines. — It  is 
the  opinion  of  many  investigators,  as  we  shall  soon  have 
occasion  to  observe,  that  either  kinsesthetic  sensations,  due 
to  orbital  movements,  or  the  impulses  to  such  movements, 
form  the  basis  of  our  visual  estimation  of  height  and  breadth. 
Thus  the  tendency  to  overestimate  the  length  of  vertical 
lines  relatively  to  horizontal  lines  (fig.  11)  has  been  attri- 
buted to  the  greater  effort  involved  in  moving  the  eyes 
in  a  vertical  than  in  a  horizontal  direction,  and  to  the 


296  EXPERIMENTAL  PSYCHOLOGY 

more  intense  kinaesthetic  experience  thus  involved  (exp. 
141). 

In  estimating  long  lines,  eye  movements  are  unavoidable, 
and  here  kinsesthesis  may  play  some  part  in  estimation. 
But  in  the  estimation  of  shorter  lines,  eye  movements  are 
negligibly  small.  Besides,  the  overestimation  of  vertical 
lines  occurs  when  they  are  presented  for  so  short  a  time  as 
to  preclude  the  possibility  of  eye  movement.  We  are  thus 
forced  to  conclude  either  that  kinassthesis,  however  im- 
portant for  the  development  of  spatial  relations  (page  279), 
is  not  essential  for  the  occurrence  of  such  illusions  in  adult 
life,  or  that  the  illusion  is  the  result -of  other  conditions, 
possibly  of  retinal  origin. 

Filled  and  Empty  Space. — The  empty  space  between  two 
dots  a  and  e  (fig.  12)  appears  to  the  eye  shorter  than  an 
equal  distance,  filled  with  dots.  Those  who  regard  kin- 


FIG.  12. 

aesthesis  as  all-important,  urge  that  in  the  former  case  the 
eye  is  free  to  move  from  one  extreme  point  to  the  other, 
while  in  the  latter  its  movement  is  repeatedly  arrested  by 
the  intervening  dots. 

Hering,  however,  offers  an  explanation  of  this  illusion 
in  purely  retinal  terms,  based  on  the  curvature  of  the  retina. 
He  reminds  us  that  the  distance  between  two  points  on  a 
flat  surface  cannot  correspond  with  the  length  of  the  arc  of 
the  curved  retina  which  receives  an  image  of  that  surface. 
Experience  has  taught  us  that  the  surface  at  which  we  are 
looking  is  flat,  and  we  estimate  the  position  of  the  points 
accordingly. 

It  is  clear  that,  were  no  allowances  made  for  the  flatness 
of  the  surface  a  e  (fig.  13),  uniocular  vision  would  locate 
the  points  a,  c,  e,  at  a',  c',  e',  respectively,  and  the  flat  surface 
would  be  seen  as  a  curved  one,  a'}  b,  c',  d,  e',  the  curve  corre- 


SIZE  AND  DIRECTION 


297 


spending  to  the  curvature  of  the  retina,  s  y  a,  and  determined 
by  the  equality  of  the  lines  a'a,  6/5,  c'/,  dd,  e'e.  But  the 
known  flatness  of  the  surface  causes  the  distance  between 
any  two  points  on  it  to  be  judged  not  on  the  basis  of  the 
curvature  of  the  retina,  but  on  the  length  of  the  chord 
drawn  on  the  retina  between  them.  Thus,  our  estimate  of 
the  unfilled  distance  between  two  points  a,  e  (fig.  12),  is 
dependent  on  the  length  of  the  retinal  chord  a  s  (fig.  13), 
and  our  estimate  of  the  broken  distances  a  6,  b  c,  c  d,  d  e,  is 
dependent  on  the  lengths  of  the  chords  a/3,  £7,  y  d,  fa. 
The  sum  of  these  four  chords  is  obviously  of  greater  length 


than  the  chord  a  e.  Hence  it  comes  about  that  an  unfilled 
distance  appears  to  be  shorter  than  an  equal  but  interrupted 
distance. 

Estimation  of  Angles. — The  eye  tends  to  overestimate 
acute  angles  and  to  underestimate  obtuse  angles  relatively 
to  one  another.  As  we  shall  see,  this  tendency  has  been 
introduced  to  explain  most  of  the  geometrical  optical 
illusions.  At  Bering's  hands  it  has  received  an  explanation 
in  purely  retinal  terms,  dependent  as  before  on  the  curvature 
of  the  retina.  According  to  this  explanation,  the  over- 
estimation  of  acute  angles  only  holds  good  for  angles  below 


298  EXPERIMENTAL  PSYCHOLOGY 

60°,  above  which  underestimation  occurs.  This  is  in  agree- 
ment with  Brentano's  statement  that  the  tendency  of  the 
error  is  to  overestimate  small  angles  and  underestimate 
large  ones. 

Wundt,  on  the  other  hand,  attributes  the  error  to  the 
greater  change  of  eye  movement  involved  in  traversing  a 
small  angle  than  in  traversing  a  large  one. 

Contrast  and  Confluence. — The  apparent  length  of  a  line 
or  the  apparent  size  or  shape  of  a  figure  is  influenced  by  the 
pressure  and  position  of  neighbouring  lines  or  figures.  The 
effect  of  the  latter  may  be  one  of  contrast  or  of  confluence. 


B 
FIG.  14. 

The  diagrams  A  and  B  in  figure  14  are  examples  of  con- 
trast. In  A  the  figures  are  of  precisely  the  same  shape ;  but 
the  upper  side  of  the  lower  figure  appears  shorter  than  that 
of  the  upper,  owing  to  its  proximity  to  the  longer  side  of  the 
upper  figure.  Similarly  the  middle  portions  of  the  two  lines 
in  B,  although  actually  equal,  appear  of  unequal  length, 
owing  to  the  different  influence  of  the  terminal  portions. 
Diagram  C,  on  the  other  hand,  is  an  instance  of  the  opposite 
effect  of  confluence.  The  middle  line  appears  longer,  when 
bounded  by  longer,  than  when  bounded  by  shorter  lines. 
These  and  other  geometrical  illusions  persist  even  when  the 


SIZE  AND  DIRECTION 


299 


nature  of  the  illusion  has  been  pointed  out.  They  cannot, 
therefore,  be  ascribed  to  errors  of  conscious  inference,  but 
are  due  to  various  factors,  chief  of  which,  perhaps,  is  our 
tendency,  in  judging  the  length  of  a  part,  to  be  inevitably 
influenced  by  the  size  of  surrounding  parts  or  of  the  entire 
figure. 

Muller -Lyer's  Figure. — The  principle  of   confluence  is 


FIG.  15. 

well  seen  in  the  Mliller-Lyer  illusion  (fig.  15),  where  the 
upper  of  the  two  horizontal  lines  appears  shorter  than  the 
really  equal  lower  line. 

The  illusion  is  more  pronounced,  the  more  prominent 
the  end  lines  are  made.  It  is  weakened  by  relative 
strengthening  of  the  horizontal  line.  If  the  end  lines  be 
lengthened,  the  illusion  increases  at  first,  but  it  diminishes 
(owing  to  the  effect  of  contrast)  when  the  relative  length 


C 


FIG.  16. 

of  the  end  lines  exceeds  a  certain  limit.  The  illusion 
diminishes  as  the  angles,  which  the  end  lines  make  with 
the  horizontal,  approach  90°. 

It  has  been  suggested  that  the  Miiller-Lyer  illusion  is 
due  to  errors  in  estimating  large  and  small  angles.  But  as 
the  illusion  persists  in  the  right-angled  form  (fig.  16),  this 
explanation  falls  to  the  ground. 

The  illusion  was  attributed  by  Einthoven  to  the  diffu- 


300 


EXPERIMENTAL  PSYCHOLOGY 


sion  circles  thrown  upon  the  retina  save  at  the  central 
region  of  distinct  vision.  He  supposed  that  the  end  lines 
would  for  this  reason  yield  blurred  dispersion  images  (fig.  17), 
and  that  we  estimate  the  lengths  of  the  horizontal  lines 
by  the  position  of  the  centres  of  the  blurred  images,  which 
are  nearer  to  one  another  in  the  one  than  in  the  other  part 


FIG.  17. 

of  the  figure.  The  validity  of  this  explanation  is  negatived 
by  the  fact  that  the  illusion  increases,  the  greater  be  the 
distance  of  the  figure  from  the  eyes, —  that  is,  the  more 
entirely  the  image  of  the  figure  fall  within  the  region  of 
distinct  vision. 

We  do  not,  as  Auerbach  believed,  necessarily  take  into 


FIG.  18. 


FIG.  19. 

account  the  length  of  imaginary  lines  between  the  end 
pieces  (fig.  18).  For  the  illusion  is  still  present  in  the 
form  shown  in  figure  19. 

There  can  be  no  doubt  that  the  main  cause  of  the 
illusion  is  due  to  the  influence  which  is  more  or  less  un- 
consciously exercised  on  the  subject  by  the  different  extents 
of  the  spaces  partially  bounded  by  the  end  lines.  The 


SIZE  AND  DIRECTION  301 

similar  effects  of  confluence  in  figure  14  G  are  referable  to 
the  same  cause. 

In  certain  primitive  peoples,  the  Miiller-Lyer  illusion 
is  found  to  be  less  marked  than  among  civilised  peoples ; 
the  former  being  able  to  take  the  end  lines  less  into  account 
and  to  attend  more  exclusively  to  the  horizontal  lines.  For  a 
like  reason  the  illusion  is  much  weakened,  if  the  colour  of 
the  end  lines  be  different  from  that  of  the  main  lines. 

Poggendorff's  Figure. — The  wrong  estimation  of  angles 
has  been  applied  to  explain  PoggendorfF s  illusion  (fig.  20), 
where  the  line  c  d  does  not  appear  to  be  a  prolongation 
of  the  line  a  I.  It  is  supposed  that  the  acute 
angle  at  I  is  overestimated,  giving  the  line  I  a 
an  unduly  horizontal  slope.  This,  however, 
is  a  very  incomplete  explanation  of  the 
illusion,  as  the  following  facts  show. 

The  illusion  decreases  with  increasing 
length  of  the  interrupted  line.  It  is  much 
reduced  when  the  figure  is  rotated  so  that 
this  line  becomes  vertical  or  horizontal. 
Again,  in  the  ordinary  position  of  the  diagram, 
the  distance  b  c  tends  to  be  overestimated, 
while  in  the  horizontal  position  of  the  inter-  20 

rupted  line  it  is  underestimated,  as  compared 
with  a  line  of  the  same  length  and  direction.  We  should 
expect  in  the  absence  of  other  influences  that  the  unfilled 
space  would  always  be  underestimated.  Its  overestimation 
when  the  figure  is  upright  is  probably  connected  with  the 
overestimation  of  vertical  spaces. 

Zollner's  Figure. — The  wrong  estimation  of  angles  has 
been  applied  to  explain  Zollner's  figure  (fig.  21).  The 
illusion  produced  by  the  cross  lines  is  at  its  height  when 
they  form  an  angle  of  30°  with  the  two  parallel  lines,  and 
is  absent  when  the  angle  reaches  70°.  It  varies  in  degree 
according  to  the  distance  of  the  pattern  from  the  eye,  there 
being  an  optimal  distance,  beyond  or  nearer  than  which  the 


302 


EXPERIMENTAL  PSYCHOLOGY 


illusion  decreases.  The  illusion  also  varies  with  the  angle 
which  the  eyes  preserve  in  regard  to  the  pattern.  It  tends 
to  disappear  upon  prolonged  fixation,  or  when  the  eyes  are 
moved  in  the  direction  of  the  parallel  lines.  It  is  much 
increased  when  the  eyes  are  moved  at  right  angles  to 
them. 

Influence  of  Suggested   Activity. — Another  explanation, 


FIG.  21. 

which  has  been  applied  by  Lipps  to  many  of  these  geometric 
illusions,  is  that  the  patterns  are  regarded  as  having  in- 
herent force  and  so  come  to  suggest  certain  mechanical 
activities,  the  radiating  lines,  for  example,  of  Bering's 
figure  (fig.  22),  suggesting  that  the  long  parallel  lines 
are  pulled  apart  from  one  another  at  their  centres.  The 
illusion,  however,  may  be  explained  by  the  overestimation 
of  acute  angles.  It  disappears  on  prolonged  fixation. 


SIZE  AND  DIRECTION 


303 


Influence  of  Perspective. — These  illusions  have  also  been 
explained  on  the  ground  of   the   perspective   which   they 


FIG.  23. 


FIG.  22. 

(consciously  or  unconsciously)  suggest.  The  apparent  per- 
spective of  simple  figures,  when  regarded  monocularly,  is 
closely  dependent  on  the 
point  of  fixation.  Thus 
in  the  lines  a  I,  c  d  (fig. 
23  A  and  B),  the  point 
c  tends  to  appear  nearer 
than  d  when  c  is  fixated, 
while  if  the  eye  wander 
to  d  the  perspective  is 
apt  to  change,  d  ap- 
pearing nearer  the  observer  than  c. 

Similarly,  the  perspective  of  the  well-known  staircase 

figure  (fig.  24)  is  apt 
to  change  according 
as  the  points  p  and 
q  are  fixated,  in  the 
former  case  appear- 
ing as  a  flight  of 
steps,  in  the  latter  as 
an  overhanging  wall 
from  which  pieces 
have  been  cut  out. 
It  is  easy,  however,  to  convince  oneself  that  these  eye 
movements  often  fail  to  reverse  the  perspective,  and  that 


FIG.  24. 


304  EXPERIMENTAL  PSYCHOLOGY 

reversals  may  occur  in  spite  of  unchanged  fixation.  Move- 
ments of  the  figure  in  a  horizontal  or  vertical  plane  have  also 
been  found  to  produce  changes  in  the  figure. 

The  General  Conditions  of  the  Geometric  Illusions. — How 
far  these  factors  of  suggested  activity  and  perspective  play 
a  part  in  the  various  illusions,  is  as  yet  undetermined.  It  is 
certain,  however,  that  if  they  are  effective  at  all,  we  are 
usually  quite  ignorant  of  their  presence.  In  general,  their 
action  is  an  instance  of  what  has  been  called  "  unconscious 
inference."  We  may  hesitate,  then,  to  term  such  factors 
psychological  lest  it  be  supposed  that  processes  of  reasoning 
are  implied.  If  they  influence  our  judgment  at  all,  they 
may  do  so  just  as  unconsciously  as  the  simpler  cerebro- 
retinal  factors  of  contrast  and  confluence  (page  298),  or  the 
still  simpler  retinal  factors  which  Hering  has  advanced 
(page  296). 

That  eye  movements  play  an  important  part  in  these 
illusions  is  gainsaid  for  various  reasons.  The  illusions  are 
not  abolished,  but,  in  many  cases  at  least,  are  increased, 
when  the  figures  are  momentarily  exhibited  by  instantaneous 
illumination  and  eye  movements  are  thus  prevented.  Most 
of  the  illusions  persist,  perhaps  reduced  in  degree,  when  the 
parts  of  the  figures  are  combined  stereoscopically.  They 
are  also  present  in  the  after-image.  Moreover,  recent 
photographic  studies  of  the  eye  movements  during  the 
regard  of  such  patterns  show  that  the  movements  are  too 
inconstant  and  variable  to  serve  as  a  basis  for  the  illusion. 
They  show,  too,  that  the  range  of  movement  is  not  greater 
in  traversing  one  of  the  figures  in  the  Miiller-Lyer  illusion 
than  in  traversing  the  other.  It  is  only  more  hampered  in 
the  arrow-head  figure.  Yet  in  this  figure  the  line  is  under- 
estimated ;  whereas  the  same  hampering  of  eye  movement 
is  used  to  explain  the  overestimation  of  filled  spaces  (fig.  12). 

Many,  at  least,  of  these  illusions  tend  to  disappear  with 
continued  practice,  even  though  the  subject  remain  ignorant 
throughout  of  the  nature  of  the  illusion.  In  the  Miiller-Lyer 


SIZE  AND  DIRECTION  305 

illusion  it  has  been  shown  that  these  effects  of  practice  only 
occur  when  the  exposure  of  the  figure  is  prolonged.  During 
momentary  exposures  the  subject  has  no  opportunity  of 
learning  to  disregard  the  end  lines  and  to  limit  his  attention 
to  the  horizontal  line  the  length  of  which  is  being 
estimated. 

The  Apparent  Size  of  the  Sun  and  Moon. — The  causes  of 
the  apparent  increase  in  size  of  the  sun  and  moon  at  the 
horizon  have  been  the  subject  of  much  controversy.  The 
illusion  varies  widely  in  different  individuals,  and  from  time 
to  time  in  the  same  individual.  The  enlargement  suddenly 
becomes  much  greater  in  the  immediate  vicinity  of  the 
horizon. 

The  usually  given  explanation  is  that  we  judge  the 
sun  and  moon  to  be  more  distant  at  the  horizon  than  at  the 
zenith,  and  that  therefore  we  infer  that  they  are  larger  at 
the  horizon.  It  is  supposed  that  one  reason  for  the  apparently 
greater  distance  of  the  sun  and  moon  at  the  horizon  lies  in 
the  atmospheric  haze  near  the  earth's  surface,  which  especi- 
ally affects  the  distinctness  and  colour  of  the  sun  and  moon 
in  that  position ;  and  that  another  reason  lies  in  the  number 
of  terrestrial  objects  intervening  between  the  observer  and 
the  sun  or  moon  at  the  horizon,  thus  providing  a  filled 
distance  which  seems  longer  than  the  equal  unfilled  distance 
between  the  observer  and  the  sun  or  moon  when  it  is  at 
the  zenith  (page  296).  It  has  also  been  thought  that  the 
absence  of  other  objects  at  the  zenith,  with  which  the 
heavenly  objects  can  be  compared,  accounts  for  the  apparent 
difference  in  their  size ;  whereas  at  the  horizon  the  sun  or 
moon  come  to  be  regarded  as  one  of  the  terrestrial  objects. 
Other  causes,  e.g.  the  size  of  the  pupil  and  movements  of  the 
lens,  have  been  also  suggested.  The  first  mentioned  of  these 
factors  is  probably  the  most  important,  and  some  of  the 
others  may  undoubtedly  play  a  subsidiary  part.  It  is  wrong, 
however,  to  say  that  they  result  in  a  judgment  that  the  sun 
or  moon  is  more  distant  at  the  horizon  than  at  the  zenith. 
20 


306  EXPERIMENTAL  PSYCHOLOGY 

As  a  matter  of  fact,  the  heavenly  bodies  at  the  horizon 
appear  to  be  not  farther  but  nearer  than  at  the  zenith. 

The  illusion  has  been  experimentally  studied  by  using  a 
mirror  so  as  to  reflect  the  sun  or  moon  at  the  horizon  to  the 
zenith,  or  vice  versd',  and  by  regarding  the  moon  at  the 
zenith  in  a  supine  position  and  the  moon  at  the  horizon 
with  the  head  inverted  between  the  legs.  The  results,  how- 
ever, are  so  contradictory  that  they  only  show  the  need  for 
further  study  of  the  many  factors  which  enter  into  the 
illusion. 

The  greater  muscular  strain  involved  in  looking  upwards 
has  also  been  advanced  as  an  explanation.  When  the  plane 
of  regard  is  raised,  the  two  eyes  converge  more  and  more, 
and  this  is  supposed  to  give  the  effect  of  increased  nearness 
or  diminished  size. 

The  form  of  the  sky  is  unquestionably  an  important 
factor  in  the  illusion.  It  usually  appears  to  be  not  hemi- 
spherical, but  flattened  at  the  horizon.  Now  objects,  shining 
through  the  vault  of  heaven,  are  regarded  as  if  they  were 
set  in  this  flattened  surface,  and  the  apparently  greater 
distance  of  the  sky  at  the  horizon  leads  to  an  apparent 
enlargement  of  the  heavenly  objects  where  they  rise  or  set. 

The  cause  of  this  flattening  of  the  sky  at  the  horizon 
has  been  much  debated.  It  has  been  referred  to  the  presence 
of  intervening  objects  on  the  earth's  surface,  to  the  results 
of  changing  the  direction  of  gaze,  and  to  the  difference  in 
coloration  of  the  sky  at  the  zenith  and  at  the  horizon, 
which  is  due  to  differences  in  transparency,  density,  and 
brightness  of  the  state  of  air.  Probably  each  of  these  is 
a  determining  factor. 

BIBLIOGRAPHY. 

E.  Hering,  Beitrage  zur  Physiologic,  Leipzig,  1861,  Hefti.  G.  Hey  mans, 
" Quantitative  Untersuch.  ii.  d.  'optische  Paradoxon,'"  Ztsch.  f.  Psychol. 
u.  Physiol.  d.  Sinnesorgane,  1895,  ix.  236.  V.  Henri,  "Illusions  et 
Hallucinations,"  L'Annde  pyycJiol.,  1896,  iii.  495  ;  ibid.,  1897,  iv.  538. 


SIZE  AND  DIRECTION  307 

W.  H.  R.  Rivers,  "On  the  Apparent  Size  of  Objects,"  Mind,  1896  (N.S.), 
v.  71  ;  Reports  of  the  Cambridge  Anthropological  Expedition  to  the  Torres 
Straits,  1901,  ii.  97;  "Observations  on  the  Senses  of  the  Todas,"  Brit. 
Journ.  of  Psychol.,  1904-5,  i.  339.  T.  Lipps,  Raumasthetik  u.  geometrisch- 
optische  Tauschungen,  Leipzig,  1897;  "Zur  Verstandigung  iiber  d. 
geometrisch  -  optischen  Tauschungen,"  Ztsch.  f.  Psychol.  u.  Physiol.  d. 
Sinnesorgane,  1905,  xxxviii.  241.  W.  "Wundt,  Die  geometrisch- optischen 
Tauschungen,  Leipzig,  1898.  E.  B.  Titchener,  Experimental  Psychology, 
New  York,  1901,  i.  expt.  xxix.  A.  H.  Pierce,  Studies  in  Auditory  and 
Visual  Space  Perception,  New  York,  1901.  E.  Reimann,  "Die  scheinbare 
Vergrosserung  d.  Sonne  u.  d.  Mondes  am  Horizont,"  Ztsch.  f.  Psychol.  u. 
Physiol.  d.  Sinnesorgane,  1902,  xxx.  1,  161 ;  ibid.,  1905,  xxxvii.  250.  E. 
Claparede,  "L'Agrandissemeut  et  la  Proximite  apparents  de  la  Lune  a 
FHorizon,"  Journ.  de  Psychol.,  1905,  v.  121.  C.  H.  Judd,  "  The  Muller-Lyer 
Illusion,"  Psychol.  Rev.,  Monograph  Suppl.,  1905,  vii.  No.  1,  55  ;  "Practice 
without  Knowledge  of  Results,"  ibid.t  185.  C.  H.  Judd,  C.  N.  M'Allister, 
and  W.  M.  Steele,  "Introduction  to  a  Series  of  Studies  of  Eye  Movements 
by  Means  of  Kinetoscopic  Photographs,"  ibid.,  1.  E.  0.  Lewis,  "The 
Effect  of  Practice  on  the  Perception  of  the  Muller-Lyer  Illusion,"  Brit. 
Journ.  of  Psychol.,  1908,  ii.  294.  G.  Dawes  Hicks  and  W.  H.  R.  Rivers, 
"The  Illusion  of  Compared  Horizontal  and  Vertical  Lines,"  ibid.,  243. 


fAjsr^Lr*^ — * 


2E: 

i^Axi_--i-^-»—  (pu^-w     >>t^«xv-»'-t-  (TU(j 


CHAPTEK    XXIII 
ON  TIME  AND   RHYTHM 

TIME. 

Temporal  Fusion. — We  have  seen  that,  during  and  after 
its  application,  a  stimulus  evokes  a  whole  series  of 
changes  in  consciousness.  We  have,  for  example,  the 
developing,  the  full  and  the  fading  sensation,  the  after- 
sensation,  and  the  primary  memory  image.  We  recognise 
that  these  stages  vary  according  to  the  kind  of  sensation, 
and  that  a  different  length  of  time  is  needful  for  the  un- 
disturbed course  of  different  sensations  in  consciousness. 

When  we  are  attending  to  such  a  series  of  changes 
evoked  by  a  momentary  stimulus, — when,  so  to  speak,  we 
are  living  jjuj^an  impression, — our  experience  of  time  is 
reduced  to  its  simplest  objective  conditions.  We  see  that 
the  apparent  length  of  the  time  depends  upon  the  number 
of  changes  in  the  state  of  consciousness,  and  upon  the  move- 
ments of  attention,  which  are  produced  by,  or  of  themselves 
produce,  those  changes. 

When  two  momentary  sounds  are  separated  by  a  brief 
interval,  of  about  550°",  the  various  changes  in  consciousness, 
to  which  we  have  just  referred,  can  be  comprehended  with- 
out difficulty  as  a  single  whole ;  they  are  subsumed  into  a 
unitary  state  of  consciousness.  The  interval  between  the  two 
sounds  appears  to  have  the  specific  character  of  "  moderate  " 
or  "  adequate "  length ;  and  the  entire  series  of  events — 

sound,  interval  and  sound — lies  within  what  has  been  called 

308 


TIME  AND  RHYTHM  309 

the  "  specious  "  or  "  sensory  "  present.  This  is  so,  because  a 
sound  needs  about  550°"  for  the  development  of  its  complete 
effect  on  consciousness.  When  a  second  sound  follows  after 
the  lapse  of  this  interval,  the  whole  series  of  events  is 
combined  and  apprehended  as  a  present  whole  with  a 
maximal  degree  of  ease  and  agreeableness.  If,  on  the  other 
hand,  the  second  sound  occur  somewhat  sooner,  the  complete 
development  of  the  effect  of  the  first  sound  is  interrupted, 
and  the  interval,  instead  of  appearing  "moderate"  or 
"adequate,"  is  now  adjudged  absolutely  "short."  Corre- 
spondingly, intervals  which  are  longer  than  the  "  adequate  " 
interval  are  termed  absolutely  "  long." 

The  interval  between  two  such  sounds  can  be  increased 
up  to  about  40  00°",  without  the  loss  of  the  power  of  com- 
bining the  successive  changes  in  consciousness  into  a  unity. 
But  the  combination,  instead  of  taking  place  passively, 
demands  an  increasing  effort  on  the  part  of  active- attention 
as  the  interval  is  lengthened.  So  long  as  the  interval 
does  not  exceed  about  4000°",  it  is  possible,  by  means  of 
strained  attention,  to  experience  the  interval  immediately  as 
a  single  percept.  Beyond  this  limit,  the  interval  is  im- 
mediately experienced  as  a  number  of  component  parts ; 
only  by  representation  or  conception  can  it  be  regarded  as 
a  single  whole.  These  limits,  however,  vary  with  different 
individuals,  and  according  to  the  nature  and  intensity  of  the 
limiting  stimuli.  They  are  materially  modified  by  certain 
drugs,  e.g.  alcohol,  opium,  and  Indian  hemp. 

The  Effect  of  Varying  the  Stimulus. — When  a  still  shorter 
time  interval,  less  than  40 0°",  is  allowed  to  elapse  between 
two  like  stimuli,  its  apparent  length  depends  upon  the 
nature  of  those  stimuli. 

An  interval  occurring  between  two  sparks  which  are 
heard^seems  shorter  than  the  same  interval  between  two 
sparks  which  are  seen.  So,  too,  an  interval  limited  by 
noises  appears  longer  than  an  equal  interval  limited  by 
electrical  stimuli.  Again,  if  three  stimuli  be  consecutively 


310  EXPERIMENTAL  PSYCHOLOGY 

presented,  the  first  two  being  light  stimuli,  the  third  being 
an  auditory  or  a  tactile  stimulus,  the  interval  between  the 
second  and  third  appears  to  be  longer  than  the  really 
equal  interval  between  the  first  and  second  stimuli.  Such 
variations  in  the  apparent  length  of  a  short  time  interval, 
according  to  the  nature  of  the  limiting  stimuli,  are  doubt- 
less dependent  upon  the  different  course  which,  as  we 
have  already  indicated,  different  sensations  pursue.  Some 
quickly  come  and  go,  others  occupy  the  attention  for  a 
longer  period.  Doubtless  the  extent  to  which  the  fading 
memory  image  of  the  first  stimulus  is  overlapped  by  the 
arrival  of  the  second  presentation  constitutes  an  important 
factor  in  the  estimation  of  short  intervals. 

Filled  and  Empty  Intervals. — The  apparent  length  of 
the  interval  elapsing  between  two  stimuli  is  influenced  by 
the  number  and  the  nature  of  experiences  occurring  during 
that  interval.  When  two  equal  intervals  are  compared  by 
a  subject,  who  is  idle  during  the  first  but  occupied  in  read- 
ing aloud  during  the  second  interval,  the  latter  almost 
invariably  appears  to  be  shorter  than  the  former,  whatever 
be  the  order  in  which  the  two  intervals  are  given.  So, 
when  two  sound  stimuli  limit  a  given  interval,  and  when 
this  interval  is  compared  with  an  equal  interval  which, 
instead  of  being  merely  limited  by,  is  also  occupied  by 
sounds,  the  "Jilled"  interval  appears  longer  than  the 
"  empty  "  interval,  the  error  of  e^Hma/EiorTmcreasing  up  to  a 
certain  point  with  the  number  of  sounds  filling  the  interval 
(exp.  143).  The  same  holds  for  visual,  and  still  more 
markedly  for  tactile  stimuli. 

The  error  is  said  to  diminish  as  the  length  of  the 
interval  increases,  and  even  to  be  reversed  when  the  times 
are  sufficiently  long.  There  are  other  complicating  con- 
ditions which  alter  the  illusion,  but  in  the  simplest  form  of 
the  experiment  the  error  we  have  described  is  that  which  is 
commonly  met  with.  The  impressions,  occupying  a  filled 
interval,  interfere  with  one  another's  free  development,  and 


TIME  AND  RHYTHM  311 

the  attention,  instead  of  being  passively  occupied  with  an 
unimpeded  experience,  is  directed  successively  to  numerous 
discrete  presentations;  whereas  in  the  case  of  the  empty 
interval,  the  impressions  which  begin  and  end  it  are  at 
liberty  to  develop  without  interruption. 

The  Filling  of  Empty  Intervals. — But,  strictly  speaking, 
an  interval  is  never  "  empty " ;  an  unfilled  interval  is  im- 
possible. Even  when  our  surroundings  are  absolutely  quiet, 
respiratory  and  cardiac  movements  are  still  occurring,  and 
the  muscles  of  the  sense  organs  or  other  parts  are  always 
undergoing  contraction.  It  has  been  supposed  that  our 
ability  to  estimate  a  fairly  long  "  empty  "  interval  depends 
on  the  kinsesthetic  and  organic  sensations  which  arise  from 
these  various  sources  and  occupy  the  interval.  It  is  true 
that,  when  subjects  are  allowed  to  use  whatever  aids  they 
like  in  order  to  remember  the  length  of  an  interval,  they  at 
first  have  recourse  to  voluntary  movements  of  the  head  or 
extremities,  or  to  voluntary  regulation  of  respiration.  They 
endeavour  to  reproduce  during  the  second  interval  the 
exact  sensations  with  which  they  have  filled  the  first 
interval,  and  they  estimate  the  relative  length  of  the  second 
interval  by  the  success  with  which  it  can  be  thus  filled. 
With  increasing  practice,  however,  subjects  gradually  dis- 
card all  such  sensory  units.  They  come  to  deem  them 
disturbing  rather  than  helpful,  and  finally  they  no  longer 
make  conscious  use  of  them  in  the  reproduction  of  intervals ; 
just  as  occurs,  indeed,  when  we  wake  each  morning  after 
a  constant  interval  of  sleep ;  or  when  through  post-hypnotic 
suggestion  we  perform  a  dictated  act  after  a  dictated 
interval. 

Experimental  Methods. — There  are  two  principal  methods 
employed  in  experiments  on  time  estimation.  The  first  is  a 
method  of  reproduction.  A  time  interval  is  presented  by 
the  experimenter ;  the  subject  has  to  repeat  this  interval  as 
correctly  as  possible  (exp.  142).  The  second  is  a  method  of 
comparison.  Two  intervals  are  presented  to  the  subject, 


312  EXPERIMENTAL  PSYCHOLOGY 

who  is  asked  to  determine  whether  one  is  longer  or  shorter 
than  or  equal  to  the  other.  These  methods  have  been 
employed  with  or  without  the  occurrence  of  a  pause  between 
the  two  intervals.  When  no  pause  is  given,  the  stimulus 
terminating  the  first  interval  also  serves  to  start  the  second. 
Sufficient  has  already  been  said,  in  the  discussion  of  the 
psycho-physical  methods,  for  us  to  realise  that  important 
differences  must  arise,  according  as  the  method  of  reproduc- 
tion or  comparison  is  used,  and  according  as  no  pause,  a 
shorter  or  a  longer  pause,  is  inserted  by  the  subject  or  by  the 
experimenter  between  the  two  intervals.  The  nature  of 
some  of  these  differences  will  be  apparent  presently. 

Tlw  Indifference  Interval.  —  By  whatever  method  we 
proceed,  one  result  of  experiment  on  time  estimation  seems 
to  stand  out  very  clearly.  Short  intervals  tend  to  be  over- 
estimated, and  long  intervals  to  be  underestimated.  There 
is  hence  an  indifference  interval,  marking  a  transition 
from  the  one  direction  of  error  to  the  other,  which  is  on 
the  average  correctly  estimated  (exp.  142).  The  earliest 
observers  found  that  the  value  of  the  indifference  point 
ranged  between  1500°"  and  3500°",  according  to  the  conditions 
of  experiment,  but  their  results  were  for  three  special 
reasons  unreliable.  The  apparatus  used  was  not  delicate 
enough  to  secure  accurate  presentation  and  measurement 
of  the  intervals,  or  to  maintain  a  uniform  intensity  of  the 
sounds  limiting  them.  In  the  second  place,  the  psycho- 
physical  method  chosen  was  not  employed  with  sufficient 
care.  Thirdly,  the  volitional  movements  made  by  the  sub- 
ject in  reproducing  a  given  interval  introduced  disturbing 
features  which  later  observers  obviated  by  using  the  method 
of  comparison.  Under  subsequently  improved  conditions  of 
experiment  the  indifference  interval  has  been  shown  to  lie 
between  700°"  and  800°".  Intervals  shorter  than  this  are 
overestimated,  those  longer  are  underestimated. 

Certain  observers  claim  that  several  other  indifference 
intervals  occur,  at  points  which  are  approximately  odd 


TIME  AND  RHYTHM  313 

multiples  of  the  interval  710°",  namely,  at  215,  3*55,  and  5 
seconds.  It  has  also  been  said  that  higher  multiples,  namely, 
64,  7*8,  9 *3,  and  10 '6  5  seconds,  are  intervals  at  which  the 
error  of  estimation  is  relatively  at  a  minimum. 

Miinsterberg,  who  believed  that  respiratory  move- 
ments are  the  basis  of  our  estimation  of  long  intervals  of 
time,  attributes  these  multiples  of  the  shortest  indifference 
interval  to  multiples  of  the  respiratory  rhythm.  Other 
organic  and  more  obscure  rhythms  have  been  also  invoked. 
The  entire  subject,  however,  needs  re-investigation. 

The  Effect  of  the  Pause. — The  comparison  of  two  intervals 
of  time  affects  and  is  affected  by  the  length  of  pause  which 
intervenes  between  them.  It  has  been  found  that,  when 
the  subject  is  allowed  to  make  what  pause  he  likes  before  he 
reproduces  a  given  interval,  the  pause  is  relatively  longest 
when  the  given  interval  is  shortest,  and  that,  as  the  interval 
which  he  has  to  reproduce  is  increased,  the  pause  absolutely 
increases  up  to  a  certain  length  of  interval,  after  which  it 
again  declines.  Attention  has  also  been  called  to  the  fact 
that,  when  the  experimenter  presents  a  pause  which  is  equal 
to  the  first  interval,  it  appears  to  the  subject  longer  than 
the  first  interval.  This  may  affect  his  judgment  when  the 
second  interval  is  presented  to  him. 

The  Effects  of  Expectation  and  Surprise. — A  further  effect 
of  an  unusually  long  or  short  pause  is  caused  by  the  feelings 
of  expectation  or  surprise.  When  a  pause  of  unusual  length 
or  shortness  occurs,  the  feelings  of  expectation  or  of  surprise 
undoubtedly  determine  our  attitude  to  the  second  interval, 
and  so  affect  our  estimation.  The  feeling,  for  example, 
which  would  be  occasioned  after  an  unduly  long  pause  by 
the  start  of  a  second  interval,  is  said  to  lead  to  under- 
estimation of  the  latter. 

These  feelings  also  play  a  more  direct  part  in  the 
estimation  of  intervals.  When  pairs  of  intervals  are  pre- 
sented, the  one  a  constant,  the  other  a  variable  interval,  and 
the  subject  has  to  judge  between  their  length,  he  becomes 


3H  EXPERIMENTAL  PSYCHOLOGY 

attuned  to  the  constant  interval,  and  to  expect  a  given 
length  of  interval  in  the  variable.  According  as  the  latter 
ends  sooner  or  later  than  he  had  been  led  to  expect,  the 
judgment  "  shorter  "  or  "  longer  "  is  recorded  of  the  variable. 
Contrast,  arising  from  side  comparisons  (pages  213,  272), 
exercises  a  similar  effect. 

The  Absolute  Impression. — So  far  we  have  presupposed 
that  time  intervals  are  "  compared,"  and  we  have  left  out  of 
account  the  possible  play  of  the  absolute  impression,  which 
dispenses  with  true  comparison.  The  absolute  judgment 
induces  us  to  disregard  the  first  of  the  two  presentations 
and  to  attend  solely  to  the  second ;  we  are  ready  to  estimate 
the  second,  even  though  we  have  lost  all  recollection  of  the 
first  (page  267).  The  first  presentation  is  only  important  in 
so  far  as  it  modifies  the  attunement  of  our  judgment. 

In  favour  of  the  existence  of  the  absolute  impression  in 
the  sphere  of  interval  comparisons,  we  have  the  following 
experimental  evidence.  Let  us  suppose  that  the  standard 
interval  measures  600°",  and  that  it  is  followed,  after  a  vary- 
ing pause  of  1*8,  14'4,  54  or  108  seconds,  by  one  of  a  series 
of  variable  intervals  ranging  between  540°"  and  660°".  Now 
the  shortest  of  these  variable  intervals,  when  judged  by  the 
subjects  on  the  ground  of  its  specific  character  (page  308), 
is  termed  absolutely  "short."  The  play  of  the  absolute 
impression,  when  pairs  of  standard  and  variable  intervals 
are  exhibited  to  the  subjects,  is  indicated  by  the  fact  that 
the  number  of  judgments  "  second  interval  shorter  "  is  found 
to  be  practically  uniform  in  spite  of  the  above  variations  in 
the  length  of  the  pause.  If  the  two  intervals,  the  standard 
and  the  variable,  be  really  compared,  we  should  expect  the 
length  of  the  pause  between  them  to  have  a  distinct  effect 
on  the  frequency  of  any  particular  judgment.  Moreover, 
the  judgments  "  second  interval  distinctly  shorter  "  are  found 
to  increase  in  frequency  with  the  length  of  the  pause.  This, 
again,  we  should  not  expect  if  a  true  process  of  comparison 
takes  place;  it  points  rather  to  reliance  on  the  absolute 


TIME  AND  RHYTHM  315 

judgment,  which  becomes  all  the  more  striking  in  effect,  the 
longer  the  pause  before  the  occurrence  of  the  time  interval 
to  which  it  refers. 

KHYTHM. 

Subjective  Accentuation  of  Rhythm. — The  appreciation  of 
regular  intervals  between  presentations  or  between  recur- 
ring groups  of  presentations,  is  the  basis  of  rhythm.  The 
presentations  may  be  tactual,  auditory,  or  kinsesthetic ;  they 
may  be  similar  or  dissimilar. 

The  simplest  material  for  rhythm  consists  of  a  series  of 
identical,  regularly  repeated,  and  equally  accented  stimuli. 
When  the  members  of  such  a  series  are  passively  attended 
to,  they  tend  to  separate  into  groups.  If,  for  example,  a 
metronome  is  rapidly  beating  with  regularity  and  uniformity, 
the  listener  who  preserves  a  passive  attitude  will  observe 
that  the  sounds  arrange  themselves  in  groups,  usually  of  two 
or  four  sounds,  the  first  member  of  each  of  which  becomes 
strongly  accented.  The  effect  of  this  subjective  accentuation 
is  that  the  intervals  between  successive  groups  appear  longer 
than  those  between  the  members  of  such  groups  (exp.  144). 

Subjective  accentuation  of  a  simple  rhythm  may  be 
changed  at  will;  subjectively  accentuated  groups  of  three 
and  of  six  may  be  realised.  Groups  of  five  are  only  with 
difficulty  obtainable,  and  cannot  easily  be  maintained. 

When  the  members  within  each  successive  group  of 
auditory  stimuli  are  of  like  loudness,  but  of  unlike  duration, 
the  longest  lasting  member  of  each  group  appears  to  be  the 
loudest ;  it  receives  the  accent,  and  is  apprehended  as  the 
first  of  each  group. 

Objective  Accentuation. — The  simplest  and  most  easily 
maintainable  rhythm,  obtained  by  objective  accentuation  of 
an  otherwise  uniform  series  of  sounds  or  movements,  is  the 
troche  -  u.  The  iambic  u  -  is  more  difficult  to  maintain, 
owing  to  its  tendency  to  pass  over  into  the  trochaic  measure. 
Similarly  the  anapaest  u  u  -  passes  over  to  the  dactyl  -  u  w. 


316  EXPERIMENTAL  PSYCHOLOGY 

That  is  to  say,  there  is  a  general  tendency  for  the  series 
to  be  grouped  so  that  the  accent  is  received  by  the  first 
member  of  every  group. 

It  will  be  noticed  that  the  objectively  accented  member 
appears  not  only  to  be  louder,  but  to  last  longer  and  to  be 
followed  by  a  longer  interval  of  time  than  the  other  members 
(exp.  145).  Thus  the  effect  of  accentuation,  whether  sub- 
jective or  objective,  is  always  to  divide  a  series  of  auditory 
stimuli  into  feet  or  bars. 

Maintenance  and  Reproduction  of  Rliythm. — The  accuracy 
with  which  a  prescribed  rhythm  can  be  maintained  also  varies 
with  the  individual  and  with  the  rate  at  which  the  presenta- 
tions succeed  one  another.  There  are  individual  differences 
in  the  rate  which  is  most  correctly  reproduced,  and  these 
have  been  attributed,  on  inadequate  evidence,  to  individual 
differences  in  the  rate  of  walking.  Eeproduction  is  stated 
to  be  most  accurate  in  the  case  of  impressions  separated  by 
an  interval  of  about  700°".  When  quicker  rates  than  this 
are  given,  they  are  reproduced  more  quickly ;  when  slower 
rates  are  given,  they  are  reproduced  more  slowly  (exp.  146). 

BIBLIOGRAPHY. 

L.  T.  Stevens,  "On  the  Time-Sense,"  Mind,  1886  (O.S.),  xi.  393. 
H.  Miinsterberg,  Beitrdge  zur  experimentellen  PsychoL,  Freiburg,  Hefte  ii.,  iv., 
1889,  1892.  F.  Schumann,  "Ueber  d.  Gedachtniss  f.  komplexe  gleiche 
Schalleindriicke,"  Ztsch.  f.  PsychoL  u.  Physiol.  d.  Sinnesorgane,  1890,  i.  75  ; 
"  Unterschiedsempfindlichkeit  f.  kleine  Zeitgrossen, "  ibid.,  1891,  ii.  294; 
"  Ueber  d.  Schatzung  kleiner  Zeitgrossen,"  ibid.,  1893,  iv.  1 ;  "Zur  Psychol. 
d.  Zeitanschaimng,"  ibid.,  1898,  xvii.  106;  "Zur  Schatzung  leerer  von 
einfachenSchallemdruckenbegrenzterZeiten,"^ic?.,1898,  xviii.  E.Meumann, 
"Beitrage  zur  Psychol.  d.  Zeitsinns,"  PhilosopJi.  Stud.,  1892,  viii.  431; 
ibid.,  1893,  ix.  264;  "  Untersuchungen  zur  Psychol.  u.  A'sthetik.  d. 
Rhythmus,"  ibid.,  1894,  x.  249  ;  "Zur  Psychol.  d.  Zeitbewusstseins, "  ibid., 
1896,  xii.  127.  T.  L.  Bolton,  "Rhythm,  "^4  wer.  Journ.  of  Psychol.,  1893,  vi. 
145,  310.  B.  Edgell,"  On  Time  Judgments, "iMd,  1903,  xiv.  418.  J.  Quandt, 
"Das  Problem  d.  Zeitbewusstseins,"  Arch.  f.  d.  ges.  PsychoL,  1906,  viii. 
(Literaturbericht),  143.  D.  Katz,  "  Experimented  Beitrage  zur  Psychol. 
d.  Vergleichs  im  Gebiete  d.  Zeitsinns, "  Ztsch.  /.  PsychoL,  1906,  xlii.  302,  414. 


CHAPTEK    XXIV 
ON   ATTENTION 

Its  Effects  on  the  Apparent  Order  of  Presentations. — When 
two  stimuli  are  simultaneously  applied  to  two  different  sense 
organs,  they  are,  as  a  rule,  not  perceived  simultaneously. 
If,  for  example,  an  auditory  stimulus  be  applied  at  the  same 
moment  as  a  visual  stimulus,  the  subject  experiences  it 
sooner  than  the  latter.  For  a  flash  of  light  to  be  perceived 
earlier  than  a  momentary  sound,  the  light  must  precede  the 
sound  by  from  60°"  to  100°";  when  this  interval  falls  to  about 
a  quarter  of  this  value,  the  sound  is  experienced  before  the 
light. 

These  conditions  are  primarily  due  to  the  longer  latency 
of  visual  as  compared  with  auditory  excitation.  They  are, 
however,  also  affected  by  the  chance  direction  of  the 
subject's  attention.  That  impression  which  he  happens  at 
the  moment  to  be  expecting  tends  to  make  its  appearance 
earlier  in  consciousness.  Perhaps  the  attention  of  some 
subjects  inclines  of  its  own  accord  towards  one  rather  than 
towards  the  other  of  two  different  simultaneous  stimuli 
which  are  expected.  This  is  a  possible  explanation  of  the 
fact  that  the  time  intervals  just  mentioned  vary  consider- 
ably in  different  individuals. 

The  time  intervals  are  materially  affected  by  volitional 
changes  of  attention.  In  one  investigation,  for  example,  it 
was  found  that,  when  attention  was  volitionally  directed  to 
the  expected  light  stimulus,  the  latter  had  to  precede  the 
sound  stimulus  by  95°"  in  order  that  it  might  be  appre- 


3*7 


3i 8  EXPERIMENTAL  PSYCHOLOGY 

bended  first,  and  that,  when  this  interval  was  reduced 
below  28°",  the  sound  was  experienced  before  the  light.  If, 
on  the  other  hand,  the  same  subject  directed  his  attention 
to  the  expected  sound,  the  latter  apparently  preceded  the 
light  stimulus,  when,  actually,  it  followed  it  by  an  interval 
of  about  5(K 

Similar  results  have  been  given  by  an  instrument 
known  as  the  "complication  clock."  This  essentially  con- 
sists of  an  index  hand,  which  is  watched  by  the  subject 
as  it  repeatedly  revolves  with  uniform  speed  before  a  white 
clock  face.  At  any  point  in  the  course  of  the  index,  deter- 
mined by  the  experimenter,  the  subject  receives  some  other 
impression,  e.g.  the  strike  of  a  bell,  a  noise,  an  electric 
shock,  or  a  combination  of  these.  After  several  observa- 
tions, the  subject  has  to  decide  the  exact  position  on  the 
dial  of  the  revolving  index  at  the  moment  when  he  heard 
the  bell  or  the  noise,  or  felt  the  electric  shock. 

The  time  displacements,  as  they  have  been  called, 
which  occur  with  this  apparatus,  resemble  those  that  we 
have  already  described.  When  the  subject's  attention  is 
especially  directed  to  the  index,  the  bell  is  heard  too  late, 
i.e.  there  is  a  positive  time  displacement.  When  he  pays 
greater  attention  to  the  bell,  it  is  heard  too  soon,  i.e.  there 
is  a  negative  time  displacement. 

As  might  be  expected,  the  position  ascribed  by  the 
subject  to  the  index  is  found  to  depend  on  the  graduation 
of  the*  clock  face.  He  tends  to  place  the  index  exactly  at 
a  mark  rather  than  between  two  marks,  and  to  place  it 
at  the  most  familiar  positions  (e.g.  corresponding  to  the 
number  12,  6,  3,  or  9  on  the  ordinary  clock  face)  rather 
than  at  an  intermediate  position. 

The  error  of  displacement  is  dependent  on  the  practice 
which  the  subject  is  allowed  and  on  the  speed  and  direction 
of  rotation  of  the  index.  According  as  the  hand  is  slowly 
or  rapidly  moving,  there  is  within  certain  limits  a  greater 
tendency  to  negative  or  to  positive  time  displacement. 


ATTENTION  319 

Between  lies  an  indifference  point,  i.e.  a  speed  of  rotation 
at  which  there  is  no  displacement.  With  an  index  length 
of  25  cm.  this  indifference  point  occurs  at  a  speed  of 
revolution  varying  between  2  and  5  sees,  according  to  the 
individual.  The  error  varies  according  to  the  number  of 
impressions  which  are  simultaneously  introduced,  and 
according  as  these  impressions  involve  the  same  or  different 
sense  organs.  Unfortunately,  a  psychological  interpretation 
of  these  and  other  results,  obtained  by  the  complication 
pendulum,  is  too  difficult  and  controversial  to  be  under- 
taken here. 

The  General  Effects  of  Attention  on  Presentations. — The 
direction  of  attention  influences  not  merely  the  order  of 
appearance  of  two  nearly  or  absolutely  simultaneous  different 
stimuli,  but,  as  we  have  seen,  it  likewise  affects  the  relative 
distinctness  and  duration  of  the  stimuli  ,and  the  absolute 
and  differential  thresholds  of  sensation.  Whether  the 
intensity  of  sensation  in  general  is  increased  by  attention 
and  reduced  by  distraction  is  a  matter  of  controversy,  which 
has  not  yet  been  finally  settled  by  experiment. 

These  general  effects  of  attention  on  the  experience  of 
a  presentation  are  most  readily  explicable  in  terms  of 
facilitation.  When  our  attention  takes  a  certain  direction, 
a  condition  is  set  up  in  which  conscious  and  unconscious 
states  tend  to  be  augmented  or  inhibited,  according  as  they 
are  favourable  or  unfavourable  to  the  continuance  of  the 
theme  of  attention.  When  we  attend  expectantly  to  a 
coming  stimulus,  everything  is  favourable  for  its  reception. 
If  the  stimulus  be  of  a  visual  kind,  the  eyes  are  already 
fixated  and  accommodated,  the  whole  muscular  and  nervous 
system  is  in  readiness  to  receive  the  stimulus.  It  is  corre- 
spondingly unprepared  to  receive  any  other  stimulus,  and 
it  resists  the  development  of  any  other  presentation.  Not 
only  are  the  appropriate  muscles  of  the  sense  organs  held 
in.  preparation,  but  the  sensory  cortical  centres  are  attuned, 
if  not  excited,  by  reason  of  the  image  of  the  coming  stimulus 


320  EXPERIMENTAL  PSYCHOLOGY 

being  held  in  consciousness.  The  preparedness  of  these 
centres  is  possibly  further  increased  by  a  drainage  into 
them  of  nervous  energy  from  the  afferent  impulses  which 
result  from  the  above-mentioned  muscular  contractions ;  but 
at  present  this  hypothesis  of  drainage  is  not  supported  by 
sufficient  physiological  evidence. 

Fluctuations  of  Attention. — When  attention  is  directed 
to  continuous  stimuli  of  feeble  intensity,  the  sensations  to 
which  they  give  rise  are  liable  to  fluctuation  (exps.  147, 
148).  If,  for  example,  a  faint  grey  band  on  a  white  or 
black  background  be  fixated,  it  comes  and  goes ;  we  are 
not  continually  conscious  of  its  presence.  These  fluctuations 
have  sometimes  been  ascribed  to  the  influence  of  cardiac  or 
respiratory  movements,  but  their  frequency  seems  to  be 
independent  alike  of  that  of  pulse  and  of  breathing.  The 
fluctuations  usually  recur  every  three  or  four  seconds ;  but 
their  periodicity  may  vary  from  a  few  seconds  even,  it  is 
said,  to  more  than  a  minute.  These  variations  depend  on 
the  absolute  strength  and  extent  of  the  stimulus,  on  its 
strength  relative  to  that  of  the  surrounding  background, 
and  on  other  factors  of  which  at  present  we  know  little. 

Similar  fluctuations  have  also  been  observed  in  the  case 
of  intermittent  auditory  stimuli,  e.g.  the  faintly  heard  ticks 
of  a  watch.  They  have  also  been  described  in  the  case  of 
weak  or  waning  continuous  sounds,  but  observers  are  not  in 
agreement  on  this  point. 

These  fluctuations  are  commonly  termed  "fluctuations 
of  attention";  but  this  begs  the  question  of  their  origin. 
From  time  to  time  attempts  have  been  made  to  explain 
these  fluctuations  in  terms  of  muscular  changes,  or  of  changes 
in  the  condition  of  adaptation,  within  the  sensory  apparatus. 
It  has  been  proved  that  the  ciliary  muscle  of  the  eye  con- 
tracts unsteadily,  and  observations  have  been  published 
which  go  to  show  that,  as  a  rule,  these  muscular  oscillations 
are  synchronous  with  the  "fluctuations  of  attention."  If 
this  be  so,  we  may  suppose  that,  as  the  lens  periodically 


ATTENTION  321 

relaxes,  the  retinal  image  is  momentarily  thrown  out  of 
focus,  its  clearly  defined  margins  giving  place  to  blurred 
diffusion  circles,  which  are  of  too  weak  intensity  to  be  seen. 
Another  explanation  is  that,  owing  to  the  well-known 
unsteadiness  of  orbital  fixation,  the  eyes  unwittingly  shift, 
whereupon  an  image  of  the  grey  band  is  cast  on  a  new 
retinal  area,  fresher  and  more  sensitive  than,  the  previous 
area,  which,  owing  to  adaptation,  no  longer  responds  to  the 
grey  stimulus. 

But  this  unsteadiness  of  the  muscles  belonging  to  the 
sense  organs  is  not  a  complete  explanation  of  the  fluctuations 
which  we  are  considering.  Visual  sensations  continue  to 
fluctuate  after  the  muscles  of  the  lens  and  pupil  have  been 
paralysed  by  atropin,  or  after  the  lens  has  been  removed  by 
operation.  The  fluctuations  recur  so  regularly  that  it  is 
unlikely  that  they  are  due  to  involuntary  eye  movements, 
and  they  persist  during  voluntary  movement  of  the  eyes. 
Auditory  sensations  fluctuate  in  persons  who,  owing  to 
disease,  have  no  tympanic  membrane  upon  which  a  periodi- 
cally contracting  tensor  tympani  muscle  might  exercise  its 
effects.  Moreover,  these  fluctuations  are  not  confined  to  the 
eye  and  ear ;  they  are  present  (although  observers  are  not 
in  agreement  here)  in  the  case  of  sense  organs  which  are 
unprovided  with  adaptative  muscular  apparatus. 

When  we  call  to  mind  that  similar  fluctuations  occur  in 
visual  after-images  (page  80),  in  memory  images  (page  148), 
in  retinal  rivalry  (page  280),  and  in  volitional  tension 
(page  194),  we  may  feel  disposed  to  believe  that  they  are 
each  the  expression  of  the  oscillatory  character  of  psycho- 
physical  processes  in  general.  The  physiological  basis  of 
such  oscillations  is  at  present  uncertain,  but  some  experi- 
mental evidence  is  claimed  in  favour  of  referring  it  to  those 
rhythmic  changes  in  blood  pressure  which  are  seen  in 
Traube-Hering  waves,  and  are  due  to  the  rhythmic  activity 
of  the  vaso-motor  centre  in  the  bulb. 

There  can  be  no  doubt  that  volitional  attention  has 
21 


322  EXPERIMENTAL  PSYCHOLOGY 

some  influence  on  the  frequency  of  these  fluctuations  of 
attention.  Its  influence  is  similarly  evident  in  the  case  of 
two  different  rival  colour  sensations,  each  derived  from  a 
different  eye ;  that  sensation,  to  which  attention  is  directed, 
being  retained  longest  and  being  made  to  predominate,  to 
the  more  or  less  complete  exclusion  of  the  other.  It  has 
been  suggested  that  this  control  by  the  attention  is  in  part 
due  to  the  muscular  accommodation  which  attention  brings 
with  it,  and  to  the  increased  sensory  experience  arising 
therefrom.  But  this,  even  if  it  be  a  partial,  is  not  the  whole 
explanation  of  the  effect,  for  the  phenomenon  does  not 
disappear  after  atropin  has  been  applied  to  one  or  both 
eyes. 

Involuntary  changes  of  fixation  do  not  afford  a  complete 
explanation  of  the  alterations  undergone  in  such  geometrical 
designs  as  figure  24  (page  303).  It  is  true  that  when  the  eyes 
are  fixated  on  the  point  p,  the  design  tends  to  take  the  form 
of  a  staircase,  and  that  when  the  eyes  are  fixated  on  the 
point  q,  it  tends  to  change  to  an  overhanging  piece  of  wall. 
Frequently,  however,  either  form  of  the  figure  may  appear, 
whichever  of  the  two  points  be  fixated,  and  the  figure  may 
alternate  between  one  form  and  another  in  the  absence  of 
any  demonstrable  eye  movement.  So  also  in  the  case  of  the 
familiar  puzzle  pictures,  where  a  man's  head,  for  example, 
has  to  be  found  amid  a  forest  of  trees,  there  is  the  same 
obscure  and  involuntary  oscillation  between  the  two  different 
possible  interpretations  of  the  same  drawing. 

Distraction. — The  effects  of  distraction  of  the  attention 
have  been  submitted  to  experimental  investigation.  It  is 
found  that  if  a  disturbance  is  either  continuous  or  regularly 
intermittent,  the  subject  soon  adapts  himself  to  it.  When 
disturbance  recurs  with  irregular  interruptions,  its  effects 
are  generally  unfavourable.  Experiment  has  shown  that, 
while  an  individual  is  maintaining  a  slow  rate  of  simple 
rhythmical  tapping,  he  is  capable  of  performing  easy  arith- 
metical calculations,  or  of  repeating  sentences  as  efficiently  as 


ATTENTION  323 

when  he  is  not  tapping.  Disturbances  enter  only  when  the 
rate  of  tapping  is  very  rapid,  or  when  he  is  desired  to 
maintain  a  complicated  rhythm.  For  a  short  time,  however, 
a  paradoxical  result  may  be  sometimes  obtained,  the  subject 
being  more  attentive,  and  executing  a  prescribed  task  more 
rapidly  in  the  presence  of,  than  in  the  absence  of,  such  a 
distracting  presentation.  That  is  to  say,  the  strain  neces- 
sary to  avoid  disturbance  directly  leads  to  increase  in  the 
degree  of  attention  given  to  the  task  prescribed. 

The  most  marked  effects  of  distraction  occur  when  the 
disturbing  and  the  disturbed  processes  are  of  closely  similar 
nature,  as  in  writing  one  poem  while  reciting  another. 

It  is  obvious  that  the  carrying  out  of  rhythmical  taps 
involves  but  dimly  conscious  processes,  which  may  be 
relegated  to  the  very  margin  of  the  field  of  attention. 
When,  however,  the  disturbing  process  is  so  important  that 
it  must  needs  occupy  the  focus  of  attention,  only  two  alter- 
natives are  possible.  Either  the  series  of  disturbing  and 
disturbed  processes  must  become  united  by  psychical  fusion 
into  a  succession  of  single  states  of  consciousness, — and  this 
is  rarely  possible, — or  the  focus  of  attention  must  oscillate 
between  the  two  tasks  while  each  is  being  carried  on.  The 
latter  is  doubtless  what  must  occur  in  the  example  just 
quoted,  wherein  the  subject  attempts  to  write  one  poem 
while  reciting  another. 

Span  of  Apprehension. — The  fact  that  a  group  of  pre- 
sentations can  be  apprehended  in  a  very  brief  and  single  act 
of  attention,  and  subsequently  analysed,  is  illustrated  in 
experiments  on  the  so-called  "  span  of  apprehension  "  (exps. 
149,  150). 

If  a  varying  number  of  points,  lines,  numbers  or  letters 
be  momentarily  exhibited  before  the  eye,  it  is  found  that 
only  a  limited  number  can  be  "  apprehended "  in  a  single 
exposure.  In  such  experiments  care  must  be  taken  that  the 
exposure  is  so  short — less  than  one  quarter  of  a  second — 
that  the  possibility  of  eye  movements  is  excluded.  The 


324  EXPERIMENTAL  PSYCHOLOGY 

eyes  should  be  accurately  fixated  on  the  empty  area  at  a 
given  signal  before  the  dots  or  lines  or  other  objects  are 
exposed  on  it.  The  various  objects  should  be  silently  ex- 
hibited and  withdrawn,  all  at  the  same  time,  and  shown  for 
a  known  and  accurately  variable  period.  Care  should  be 
taken  that  the  illumination  of  the  objects  does  not  appreci- 
ably disturb  the  state  of  retinal  adaptation  of  the  subject. 

With  these  precautions  it  is  found  that  only  about  five 
separate  impressions  (points,  lines,  numbers  or  letters)  can 
be  counted  after  they  have  been  momentarily  seen.  When 
short  words,  of  three  or  four  letters,  are  substituted,  again 
only  about  five  words  can  be  apprehended.  That  is  to  say, 
only  five  separate  units  can  be  analysed  from  the  experience 
resulting  from  a  very  brief  act  of  attention.  Experiments 
have  shown  that  the  number  of  units  which  can  be  analysed 
by  different  subjects  is  related  to  individual  differences  in  the 
duration  and  vividness  of  the  memory  after-image. 

Each  of  these  units  may,  as  is  clearly  the  case  with  short 
words,  be  composed  of  smaller  units,  which  are  psychically 
apprehended  as  a  larger  unit,  just  as  the  larger  units  them- 
selves are,  within  the  limits  of  the  "  span  of  apprehension," 
apprehended  as  a  unitary  state  of  consciousness.  It  is 
obvious  that  increasing  practice  may  permit  of  the  forma- 
tion of  still  more  complex  units. 

Investigations  have  been  likewise  made  with  regard 
to  groups  of  successive  instead  of  simultaneous  stimuli. 
Inquiries  have  been  directed  to  the  number  of  metronome 
taps  which  a  subject,  without  counting,  can  recognise  as 
a  whole.  It  is  found  that  when  the  taps  succeed  one 
another  every  quarter  of  a  second,  the  subject  can 
just  apprehend  groups  of  eight.  If  one  group  (the  first 
member  of  which  is  accented  by  a  bell)  consists  of  eight 
taps,  while  another  group  (similarly  accented)  consists  of 
seven  taps,  the  subject  can,  without  counting,  distinguish 
the  one  group  from  the  other;  but  beyond  groups  of  eight 
taps,  his  judgments  are  unreliable. 


ATTENTION  325 

The  Measurement  of  Attention. — Four  principal  methods 
have  been  applied  or  suggested  to  this  end.  In  the  first  the 
rate  of  fluctuations  of  minimal  stimuli  (page  320)  is  observed 
under  different  conditions  of  attention.  The  second  method 
consists  in  measuring  the  organic  concomitants  of  attention 
(page  333).  In  the  third  method  the  efficiency  with  which 
a  certain  test  is  performed  is  taken  as  a  measure  of  the 
attention.  Among  the  tests  which  different  observers  have 
chosen  for  this  purpose,  are  the  spatial  and  differential 
thresholds,  reaction  times,  the  counting  or  marking  of  dots 
during  short  exposures,  learning,  and  the  letter-erasing  test. 

In  all  three  methods,  what  is  measured  is  not  the  process, 
but  a  product,  or  a  concomitant,  of  attention.  Psychologically 
they  are  alike  untrustworthy,  in  so  far  as  they  neglect  the 
record  of  introspection  by  the  subject  in  regard  to  the  state 
of  his  attention  or  the  distinctness  of  what  he  is  attending 
to.  None  of  them  provides  a  sufficiently  reliable  measure 
by  which  the  attention  of  an  individual  may  be  compared 
at  different  times,  or  with  the  attention  of  other  individuals. 
A  fourth  method,  for  which  greater  promise  has  been  claimed, 
consists  in  determining  by  numerous  experiments  the 
various  strengths  of  a  distracting  stimulus  necessary  for 
producing  definite  amounts  of  deterioration  in  the  efficiency 
with  which  the  subject  executes  a  given  task;  and  in 
correlating  the  effects  of  such  distracting  stimuli  with  the 
introspective  data  afforded  by  the  subject. 

It  is,  however,  doubtful  whether  attention  in  general 
is  susceptible  of  measurement.  This  doubt  involves  a 
thorough-going  inquiry  into  the  fundamental  nature  of 
attention, — a  problem  into  which  we  cannot  enter  here. 

BIBLIOGRAPHY. 

J.  R.  Angell  and  A.  H.  Pierce,  "Experimental  Researches  on  the 
Phenomena  of  Attention,"  Amer.  Journ.  of  PsyclwL,  1892,  iv.  528.  A.  J. 
Hamlin, "  On  Least  Observable  Differences  between  Stimuli,"  Amer.  Journ.  of 
Psychol.j  1894,  vi.  564.  W.  Heinrich,  "  Zur  Erklarung  d.  Intensitatsschwan- 


326  EXPERIMENTAL  PSYCHOLOGY 

kungen  ebenmerkliclier  opt.  u.  akust.  Eindriicke,"  Bullet,  internal,  de 
VAcad.  des  Sciences  de  Cracovie,  1898,  365  ;  "  De  la  Constance  de  Perception 
des  Tons  purs  a  la  Limite  de  1'Audibilite,"  ibid.,  1900,  37.  J.  W.  Slaughter, 
"The  Fluctuations  of  the  Attention  in  some  of  their  Physiological  Relations," 
Amer.  Journ.  of  Psychol.,  l$QQ,  xii.  313.  W.  M'Dougall,  "The  Physiological 
Processes  in  the  Attention  Process,"  Mind,  1902  (N.S.),  xi.  316  ;  1903  (N.S.), 
xii.  289,  473;  1906  (N.S.),  xv.  329.  W.  Wirth,  "  Zur  Theorie  d. 
Bewusstseinsumfanges  u.  seiner  Messung,"  Philosoph.  Stud.,  1902,  xx.  487. 
F.  G.  Bonser,  "A  Study  of  the  Relations  between  Mental  Activity  and  the 
Circulation  of  the  Blood, "  Psychol.  Rev.,  1903,  x.  120.  W.  Wundt,  Grundziige 
d.  physiol.  Psychol.,  Leipzig,  1903,  iii.  67,  351.  B.  Hammer,  "Zur  experi- 
ment. Kritik.  d.  Theorie,  d.  Aufmerksamkeitsschwanknngen,"  Ztsch.  f. 
Ptychol.  u.  Physiol.  d.  Sinnesorgane,  1905,  xxxvii.  363;  Ztsch.  f.  Psychol., 
1906,  xii.  48.  W.  Peters,  "  Aufmerksamkeit  u.  Zeitverschiebung  in  d. 
Auffassung  disparater  Sinnesreize,"  Ztsch.  f.  Psychol.  u.  Physiol.  d.  Sinnes- 
organe, 1905,  xxxix.  401.  C.  E.  Ferree,  "An  Experimental  Examination  of 
the  Phenomena  usually  attributed  to  Fluctuations  of  the  Attention,"  Amer. 
Journ.  of  Psychol.,  1906,  xvii.  81  ;  ibid.,  1908,  xix.  58.  W.  B.  Pillsbury, 
Attention,  London,  1908. 


CHAPTEK    XXV 
ON   FEELING1 

The  Determinants  of  Feeling. — The  way  in  which  any 
given  presentation  affects  us  is  partly  determined  by  the 
nature  of  that  presentation.  Certain  odours,  tastes,  shapes, 
and  situations,  certain  combinations  of  colour  or  of  tone, 
seem  inherently  and  almost  universally  pleasant,  beautiful, 
exciting,  comic,  or  the  reverse,  or  indifferent. 

We  may  roughly  investigate  this  relation  between  the 
cognitive  and  the  affective  modes  of  consciousness,  by 
taking  into  our  consideration  objects  and  situations  which 
we  experience  in  daily  life ;  thus  we  may  note  the  propor- 
tions of  lines  or  the  combinations  of  colours,  which  are 
preferred  for  everyday  use.  We  may  supplement  this 
"  method  of  observation "  by  a  "  method  of  production,"  in 
which  subjects  are  asked  to  produce  those  proportions, 
combinations,  or  forms  which  they  consider  pleasant,  beauti- 
ful, etc. 

The  broad  relations  thus  shown  to  subsist  between 
sensory  (or  ideal)  presentation  and  feeling  are  by  no  means 
the  sole  determinants  of  the  affective  outcome  of  such  a 
presentation.  The  total  state  of  the  subject  has  also  to  be 
taken  into  consideration, — his  past  education  and  experience, 
his  proneness  to  suggestion  and  to  adaptation,  his  present 
mood  or  train  of  thought,  and  other  like  factors.  It  is  not 
sufficient  to  have  determined  statistically  that  a  given 
stimulus,  or  rather  its  percept  or  idea,  is  pleasant,  indifferent, 

1  The  word  is  used  very  loosely  as  a  convenient  heading  to  this  chapter. 


327 


328  EXPERIMENTAL  PSYCHOLOGY 

or  depressing.  Investigations  must  be  so  planned  as  to 
examine  alike  the  individual  subjective,  as  well  as  the 
objective,  determinants  of  feeling  in  an  adequately  psycho- 
logical manner. 

Experimented  Procedure. — As  in  so  many  other  themes 
of  research  in  experimental  psychology,  investigations  upon 
feeling  have  been  for  the  most  part  confined  to  the  use  of 
simple  objects.  In  experimental  aesthetics,  for  example, 
rectangles,  triangles,  curved  or  straight  lines,  or  simple 
combinations  of  colour,  form  the  chief  material  that  has  as 
yet  been  used.  The  advantage  of  employing  such  simple 
objects  lies,  in  the  first  place,  in  the  fact  that  the  aesthetic 
or  other  relations  are  simpler,  and  that  introspection  is 
proportionately  easier ;  and,  in  the  second  place,  in  the  fact 
that  we  can  prepare  a  graded  series  of  any  particular 
variable.  For  instance,  a  series  of  equally  graded  shades 
of  a  given  colour  may  be  obtained;  or  a  given  line  may 
be  made  to  vary  in  length,  in  curvature,  in  inclination,  or 
in  thickness,  or  simultaneously  in  two  or  more  of  these 
directions. 

Such  changes  in  the  object  are  usually  effected  by  as 
many  successively  different  exposures,  but  they  may  some- 
times be  effected  continuously  during  a  prolonged  exposure. 
Thus  by  an  arrangement  similar  to  that  described  in  experi- 
ment 141,  a  given  object,  e.g.  a  rectangle,  or  the  body  of  a 
dachshund,  may  be  gradually  elongated  or  shortened.  In 
these  experiments  the  point  of  maximal  feeling  may  be 
estimated  by  the  method  of  production  (page  202),  or  by 
the  method  of  comparison,  which  we  shall  describe 
immediately. 

When  a  series  of  objects,  varying  in  one  particular 
direction,  has  been  prepared,  they  may  be  exhibited  to  the 
subject  simultaneously  or  successively;  they  may  be  ex- 
hibited all  together,  singly,  or  in  groups  of  two. 

The  Method  of  Choice. — When  many  of  the  objects  are 
simultaneously  exhibited,  we  employ  the  "  choice  "  method. 


FEELING  329 

The  subject  is  asked  to  choose  the  object  which  most  excites 
in  him  that  feeling  which  is  under  investigation.  It  is  a 
cruder  method  and  obviously  gives  less  valuable  psycho- 
logical information  than  either  of  the  following. 

The  Method  of  Comparison. — This  is  a  method  of  paired 
exposures,  the  members  of  each  pair  being  exhibited  success- 
ively or  simultaneously,  and  the  subject  comparing  the 
degree  of  feeling  produced  by  each.  It  is  really  only  a 
modification  of  the  constant  method  (page  210).  One  member 
of  each  pair  is  the  constant  or  standard  throughout  a  given 
number  of  paired  exposures,  while  the  other  is  variable. 

When  the  members  of  each  pair  are  simultaneously 
exposed,  the  so-called  space  error  may  be  measured ;  when 
they  are  successively  exposed,  both  the  space  and  the  time 
errors  may  be  determined.  The  subject's  answers  may  be 
graded  thus — "  much  pleasanter,"  "  pleasanter,"  "  doubtful," 
"  indifferent,"  "  more  unpleasant,"  "  much  more  unpleasant "  ; 
or  other  variations  may  be  introduced  which  were  suggested 
when  we  were  describing  the  constant  method. 

The  standard  may  remain  the  same  throughout  different 
groups  of  experiments  repeated  at  different  times.  In  this 
case  the  standard  should  be  one  which  is  known  to  excite 
only  a  moderate  degree  of  the  feeling  under  investigation. 
Or  the  standard  may  be  systematically  varied  in  different 
groups.  Thus,  if  a  series  of  graded  objects  be  denoted  by 
the  letters  A,  B,  C,  D,  .  .  .  K,  the  first  experiment  may 
consist  of  the  standard  A  simultaneously  presented  with 
B,  then  with  C,  then  with  D,  etc. ;  in  the  second  experiment 
B  will  be  standard,  and  will  be  presented  with  A,  C,  D,  etc. ; 
in  the  third  experiment  C  becomes  the  standard,  and  is 
successively  presented  along  with  A,  B,  D,  etc. ;  and  so 
on  until  each  member  of  the  series  has  been  in  turn  a 
standard.  A  second  series  of  experiments  may  be  repeated 
in  inverse  order,  K  being  the  first,  and  A  the  last  used 
standard.  Thereupon  a  third  and  fourth  series  should  be 
obtained,  in  which  the  standard  lies  on  the  opposite  side  of 


330  EXPERIMENTAL  PSYCHOLOGY 

the  variable  to  that  which  it  had  occupied  in  the  previous 
two  series. 

The  Method  of  Single  Exposures. — In  this  method,  often 
called  the  "  serial  method,"  the  objects  are  shown  singly  in 
regular  or  irregular  order,  and  the  judgments  of  the  subject 
are  absolute  instead  of  relative.  They  may  be  recorded 
thus—"  very  beautiful,"  "  beautiful,"  "  doubtful,"  "  in- 
different," "ugly,"  "very  ugly."  This  method  serves  as  a 
valuable  control  over  the  method  of  comparison.  For  when 
two  objects  A  and  B  are  compared,  although  A  may  be 
liked  better  than  B,  yet  absolutely  (apart  from  its  relation 
to  B)  the  subject  may  be  indifferent  to  it,  or  may  even 
dislike  it. 

The  Influence  of  Association. — An  important  advantage 
obtained  by  the  use  of  simple  objects  consists,  as  we  have 
already  said,  in  the  facilitation  of  introspection.  The 
simpler  the  object,  the  more  readily  can  the  subject 
determine  the  content  of  consciousness  and  the  degree  to 
which  his  feelings  are  influenced  by  the  object  itself  or 
by  the  associated  experiences  which  the  object  arouses. 
The  individual  differences  that  have  been  experimentally 
demonstrated,  both  as  regards  the  degree  of  feeling  and  the 
nature  and  number  of  associated  experiences  directly  or 
indirectly  aroused  by  the  object,  are  astonishingly  great. 
A  given  oblong  figure,  for  example,  may  be  preferred  by  a 
subject  because  its  shape  suggests  that  of  a  room  which  is 
full  of  pleasant  associations;  or  a  uniform  field  of  colour 
may  appear  unpleasant  because  it  suggests  the  dress  of 
some  person  who  is  disliked.  In  some  cases,  at  least,  these 
diverse  effects  of  association  become  less  obtrusive  as  the 
subject  becomes  accustomed  to  the  experiments,  until 
finally  the  feelings  directly  attached  to  the  simple  objects 
which  are  exposed  to  him  are  free  from  disturbance  and 
open  to  examination.  In  other  individuals,  such  associations 
play  little  or  no  part  in  their  preferences.  They  like  a 
colour  because  subjectively  it  is  warm,  stimulating,  or 


FEELING  331 

soothing;  or  because  objectively  it  is  a  "pure"  colour. 
Others,  again,  are  found  to  treat  the  colour  as  if  it  were  a 
living  subject.  They  dislike  it  because  it  is  unfriendly  or 
dishonest,  or  like  it  on  account  of  its  frankness  or  joviality. 

The  Duration  of  Exposure. — The  effect  of  duration  of 
exposure  upon  the  feeling  aroused  by  an  object  is  capable 
of  experimental  investigation.  It  is  usually  found  that  up 
to  a  certain  length  of  exposure  the  feeling  grows  in  degree, 
but  that  with  longer  exposures  this  no  longer  holds,  the 
character  of  the  feeling  changing,  and  a  previously  pleasing 
experience  even  becoming  displeasing. 

^Esthetic  effects  have  been  obtained  even  with  very 
short  exposures,  during  which,  it  may  perhaps  be  assumed, 
the  subject  has  no  opportunity  of,  so  to  speak,  "  living  into  " 
the  experience.  Doubtless  with  longer  exposures,  this 
factor  of  "  empathy," l  as  Lipps  insists,  plays  an  important 
part.  The  subject  feels  in  himself  the  suggestions  of  strain, 
movement,  or  rest  in  the  object,  and  makes  them  part  of 
himself  (page  302).  The  aesthetic  value  of  the  object 
depends  much  on  the  nature  of  these  suggestions.  By 
further  modifications  in  experiment,  and  by  appeals  to 
introspection,  we  may  hope  to  examine  the  truth  of  other 
theories  that  have  been  advanced. 

It  remains  for  future  experimental  investigation  to 
determine  the  precise  effects  of  simultaneous  and  successive 
contrast  of  feeling,  and  the  effects  of  summation  and  com- 
pensation of  feelings.  A  beginning  has  been  already  made. 
Individual  differences,  too,  have  been  investigated  with 
regard  to  the  uniformity  of  judgments  on  different  days, 
the  effect  of  previously  exposed  objects,  and  the  influence 
of  past  occupation  and  past  judgments  on  present  feeling. 
Into  these  and  other  results  it  is  impossible  here  to  enter. 
They  may  be  regarded  as  pioneer  work  paving  the  way  to 
more  extended  observations. 

1  Professor  James  Ward  suggests  to  me  this  convenient  translation  of  the 
German  Einfulilung. 


332  EXPERIMENTAL  PSYCHOLOGY 

The  Motor  Manifestations. — Experimental  psychology 
has  long  devoted  itself  to  analysing  the  movements  whereby 
feelings  gain  expression,  and  to  determining  what  relation 
exists  between  the  various  feelings  and  their  accompanying 
movements. 

These  movements  may  be  broadly  classed  as  organic  and 
skeletal ;  the  former  being  carried  out  by  the  muscles  of  the 
abdominal  and  thoracic  viscera  and  by  the  blood  vessels,  the 
latter  by  the  voluntary  muscles  of  the  body. 

Organic  Movements. — Changes  in  the  rate  and  depth  of 
respirations  may  be  readily  studied  by  means  of  the  pneumo- 
graph  (exp.  151) ;  changes  in  the  rate  and  force  of  the 
heart  beat  by  applying  the  sphygmograph  to  the  pulse 
(exp.  152),  or  the  plethysmograph  to  a  limb  (exp.  153); 
changes  in  the  volume  of  a  limb,  due  to  changes  in  the 
heart  beat,  or  to  active  dilatation  or  constriction  of  the 
blood  vessels,  are  revealed  by  the  plethysmograph ;  changes 
in  the  blood  pressure  by  the  sphygmograph,  the  plethysmo- 
graph, or  the  sphygmomanometer. 

Attempts  have  been  made  to  reduce  experimental  con- 
ditions to  the  utmost  simplicity  by  blindfolding  the  subject, 
enjoining  him  to  cultivate  a  state  of  reverie,  and  then 
applying  a  sensory  stimulus  which  is  calculated  to  produce 
in  him  a  definite  change  of  feeling.  The  organic  movements 
which  are  set  up  in  the  subject  are  carefully  noted  by  the 
experimenter,  who  correlates  them  with  the  changes  in 
feeling  to  which  the  stimulus  has  given  rise. 

As  a  general  rule,  a  pleasant  stimulus,  e.g.  a  pleasant 
odour  or  sound,  causes  an  increased  volume  of  the  limb 
as  determined  by  the  plethysmograph.  This  rise  in  volume 
is  usually  preceded  by  an  initial  fall.  Unpleasant  stimuli 
produce  a  simple  fall  in  volume.  The  respiratory  changes 
are  found  to  vary  widely  in  different  individuals.  Pleasant 
tastes  quicken  the  pulse  and  raise  its  height,  the  increase 
being  most  evident  in  the  case  of  very  faintly  pleasant 
stimuli.  Pleasant  tones  and  colours,  on  the  other  hand, 


FEELING  333 

are  said  to  slow  the  pulse.  According  to  some  observers, 
the  pulse  wave  diminishes  in  height  and  in  length  when  the 
stimulus  is  unpleasant.  But  according  to  others,  the  pulse 
almost  invariably  quickens  with  the  exhibition  of  unpleasant 
stimuli,  its  frequency  increasing  with  the  degree  of  un- 
pleasantness. We  shall  later  recall  attention  to  this 
divergence  between  the  results  obtained  by  different  in- 
vestigators and  from  different  subjects. 

While  the  subject  is  attending  to  an  easy  task,  his  pulse 
is  unchanged  or  becomes  slower,  and  his  respirations  grow 
shallower.  But  in  strained  attention,  the  pulse  always 
quickens,  its  force  and  the  volume  of  the  arm  diminish, 
while  the  changes  in  breathing  vary  according  to  the 
individual.  If,  during  such  a  state  of  tension,  either  a 
pleasant  or  an  unpleasant  stimulus  be  exhibited  to  the 
subject,  the  pulse  always  slows ;  in  other  words,  the  signs 
of  relaxed  attention  replace  those  of  pleasure  or  displeasure, 
although  the  feelings  of  pleasure  or  displeasure  are  never- 
theless present. 

There  is  much  other  experimental  evidence  which  shows 
how  dependent  the  organic  changes,  that  accompany  a 
change  of  feeling,  are  upon  the  nature  of  previous  or 
simultaneous  affective  states.  Thus  it  appears  that,  while 
unpleasant  stimuli  are  accompanied  by  a  diminution,  and 
while  expectation  is  accompanied  by  an  increase  in  the 
volume  of  the  arm,  the  latter  may  remain  unchanged  when 
expectation  and  unpleasantness  coexist. 

Sensations  of  pain  quicken  the  rate  of  the  pulse  and 
usually  that  of  respiration.  The  pulse  is  slowed  in  joy  and 
grief,  when  accompanied  by  excitement  and  expectation; 
the  latter  quicken  the  pulse.  At  the  very  onset  of  fear,  the 
pulse  is  said  to  increase  in  force  and  to  diminish  in  rate. 

Skeletal  Movements. — Similar  simple  methods  of  experi- 
ment have  been  applied  to  determine  the  effects  of  changes 
of  feeling  upon  the  contraction  of  skeletal  muscles.  For 
this  purpose  the  muscles  of  the  limbs  have  been  examined 


334  EXPERIMENTAL  PSYCHOLOGY 

both  in  the  relaxed  and  in  the  contracted  state.  In  experi- 
ments on  relaxed  muscles,  the  arm  is  supported  comfortably 
upon  an  apparatus  that  resembles  the  planchette,  capable, 
that  is,  of  recording  the  slightest  movement  involuntarily 
imparted  to  it  by  the  arm  (exp.  154).  Some  observers  have 
found  that  pleasant  and  unpleasant  stimuli  unconsciously 
produce  movements  of  extension  and  flexion  respectively. 
It  is  asserted,  however,  that  in  certain  individuals  precisely 
the  opposite  relation  exists. 

The  effect  of  such  stimuli  upon  already  contracted 
muscles  may  be  investigated  by  observing  the  record  of 
a  dynamometer,  which  is  grasped  as  forcibly  as  possible  by 
the  muscles  under  investigation,  in  the  presence  or  in  the 
absence  of  various  stimuli  (exp.  155).  In  place  of  the  ordi- 
nary dynamometer  a  spring  balance  may  be  used.  This  is 
suspended  vertically  and  is  pulled  upon  by  two  fingers  of 
the  comfortably  supported  limb.  The  movements  of  the 
index  of  the  balance  may  be  easily  transferred,  by  means  of 
cords,  pulleys,  and  levers,  to  a  travelling  smoked  surface. 
The  record  obtained  under  ordinary  conditions,  when  the 
blindfold  subject  tries,  say  for  a  minute,  to  maintain  a  state 
of  maximal  contraction  is  an  obliquely  descending  almost 
unbroken  line.  When,  however,  a  stimulus  is  exhibited  to 
the  subject,  during  this  experiment,  the  record  is  altered 
according  to  the  feeling  produced.  A  very  pleasant  stimulus 
usually  causes  an  initial  drop  followed  by  a  significant  rise 
in  the  tracing;  after  which  the  tracing  gradually  falls, 
though  maintaining  a  higher  level  than  usual.  A  very  un- 
pleasant stimulus,  on  the  other  hand,  causes  a  decided  fall  in 
the  tracing,  after  which  there  is  a  gradual  fall,  the  tracing 
maintaining  a  lower  level  than  usual. 

The  initial  drop  occurs  both  with  pleasant  and  un- 
pleasant stimuli,  and  with  stimuli  which  are  of  indifferent 
character.  But  while  in  the  first  and  last  cases  it  is 
transient  and  is  perhaps  due  to  a  distraction  of  the  sub- 
ject's attention,  in  the  second  not  only  is  the  effect  more 


FEELING  335 

marked,  but  it  persists  to  some  extent  as  long  as  the  un- 
pleasantness remains. 

Attempts  have  been  made  to  determine  whether  any 
relation  exists  between  the  appreciation  of  beauty  in  the 
form  of  an  object  and  the  eye  movements  which  take  place 
during  regard  of  it.  The  eye  movements  have  been  recorded 
by  photographic  methods,  and  the  results  have  shown  that 
the  aesthetic  value  of  curved  outlines  bears  no  relation  to  the 
movement  of  the  eyes  as  they  wander  over  the  figure.  In 
regarding  a  beautiful  vase,  the  eyes  do  not  follow  its  sinuous 
contour,  but  they  move  irregularly  in  zigzag  fashion  over 
the  surface  of  the  object. 

The  Relation  between  Feelings  and  their  Expression. — 
The  interpretation  of  these  investigations  of  the  relation 
between  feelings  and  their  expression  has  been  often  coloured 
by  the  preconceived  theories  held  by  the  investigators.  We 
may  recall  the  fact  that  Wundt,  analysing  certain  records, 
discovers  in  them  evidence  that  all  feelings  may  be  resolved 
in  the  directions  of  pleasure  or  displeasure,  excitement  or 
depression,  strain  or  relief ;  that  Eoyce  would  substitute  for 
this  tri-dimensional  theory  a  bi-dimensional  theory  of  feel- 
ing, comprising  pleasantness  or  unpleasantness,,  and  restless- 
ness or  quiescence;  while  Titchener  insists  that  all  such 
"  feelings,"  save  pleasantness  and  unpleasantness,  can  be 
reduced  to  sensorial  experience  in  terms  of  kinsesthesis  or 
general  sensibility  (ccensesthesis),  and  that  pleasantness  and 
unpleasantness  are  the  sole  elements  of  feeling. 

Whatever  be  our  limitation  or  definition  of  feeling,  we 
have  sufficient  evidence  to  indicate  that  pleasure  and  dis- 
pleasure are  independent  of  afferent  impulses  derived  from 
organic  or  skeletal  movements.  No  circulatory,  respiratory, 
or  visceral  disturbances  have  been  hitherto  identified  as 
being  indispensable  concomitants  of  pleasure  and  dis- 
pleasure. On  the  contrary,  we  have  shown  (page  333)  that 
pleasure  and  displeasure  may  exist  in  the  absence  of  such 
changes. 


336  EXPERIMENTAL  PSYCHOLOGY 

Nevertheless  we  must  regard  the  various  organic  and 
skeletal  movements,  which  we  have  described,  as  closely 
associated  with  the  presence  of  the  states  of  pleasure  or 
displeasure  themselves.  For  when  a  subject  has  been 
anaesthetised  by  laughing  gas,  the  organic  movements  which 
would  normally  be  produced  by  pleasant  or  unpleasant 
stimuli  do  not  occur ;  and  in  partial  anaesthesia,  the  move- 
ments vary  in  degree  with  the  depth  of  the  anaesthesia.  They 
have  been  produced  during  hypnosis  by  suggestion  of  the 
appropriate  feeling. 

Accordingly,  we  may  consider  the  organic  and  skeletal 
movements  not  as  determinants  of  the  degree  of  pleasure 
and  displeasure,  but  rather  as  the  outcome  of  those  central 
changes  (producing  excitement,  effort,  or  strain)  which  accom- 
pany and  follow  pleasure  or  displeasure.  The  sensory  im- 
pulses, derived  from  such  movements,  doubtless  reinforce 
the  central  changes  in  question.  Adopting  this  view,  we 
may  with  far  greater  probability  refer  the  influence,  which 
a  pleasant,  indifferent,  or  unpleasant  stimulus  has  upon 
muscular  effort,  to  changes  in  central  excitement  or  strain 
rather  than  to  the  pleasant,  indifferent,  or  unpleasant  feel- 
ing to  which  it  gives  rise. 

We  regard  pleasure,  indifference,  and  displeasure  as 
psychical  attributes  of  all  states  of  consciousness,  whether 
predominantly  intellectual,  conative,  or  affective,  while  we 
regard  involuntary  movement  as  especially,  although  not 
solely,  characteristic  of  affective  states.  As  to  the  time 
relation  between  affection  and  the  organic  and  other  move- 
ments, it  is  extremely  difficult  to  obtain  satisfactory  experi- 
mental evidence.  The  results  of  different  investigators  are 
hopelessly  discordant,  and  until  these  and  other  results  are 
in  better  agreement,  it  is  impossible  to  consider  their 
relevance  to  the  James-Lange  theory,  which  identifies 
emotion  with  organic  sensation. 


FEELING  337 


BIBLIOGRAPHY. 

T.  Fechner,  Vorschule  dcr  ^Esthetik,  Leipzig,  1876,  i.  C.  Fere,  Sensation 
ct  Mouvcment,  Paris,  1889,  1900.  A.  Lehmann,  Die  kdrperlichen  Ausser- 
ungen  psychischer  Zustdnde  (deutsche  Uebersetz.),  Leipzig,  Tie.  1,  2,  1899, 
1901.  Larguier  des  Bancels,  ' '  Les  Methodes  de  1'Esthetique  experimentale, " 
L' Annee psychol.,  6me  Annee,  1900, 144.  G.  M.  Stratton,  "Eye-movements 
and  the  Esthetics  of  Visual  Form,"  Philosoph.  Stud.,  1902,  xx.  336.  P. 
Zoneffu.  E.  Meumann,  "Ueber  Begleiterscheinungenpsycliischer  Vorgangein 
Atem  u.  Puls,"  Philosoph.  Stud.,  1902,  xviii.  45.  H.  C.Stevens,  "A  Plethys- 
mographic  Study  of  Attention,"  Amer.  Journ.  of  Psychol.,  1905,  xvi.  409. 
K.  Gordon, "Ueber  das Gedachtniss fiir  affektiv  bestinmite  Eindrlieke,"  Arch, 
f.  d.  ges.  Psychol.,  1905,  iv.  437.  M.  Kelchner,  "  Die  Abhangigkeitd.  Atem- 
u.  Pulsveranderung  vom  Reiz  u.  vom  Gefiihl,  ibid.,  1905,  v.  1.  L.  J.  Martin, 
"An  Experimental  Study  of  Fecliner's  Principles  of  ^Esthetics,"  Psychol. 
Rev.,  1906,  xiii.  142.  P.  Souriau,  "Revue  d'Esthetique,"  L'Annee  psychol., 
12me  Annee,  1906,  406.  0.  Kiilpe,  "  Der  gegemvartige  Stand,  d.  experi- 
mentellen  Astlietik,"  Bericht  iiber  d.  II.  Kongrcss  f.  experim.  Psychol., 
Leipzig,  1907,  1.  E.  Bullough,  "  On  the  Apparent  Heaviness  of  Colours, " 
Brit.  Journ.  of  Psychol.,  1906-8,  ii.  Ill ;  "On  the  'Perceptive  Problem'  in 
the  ^Esthetic  Appreciation  of  Single  Colours,"  ibid.,  406.  F.  Peterson  and 
C.  G.  Jung,  "  Psycho -physical  Investigations  with  the  Galvanometer  and 
Pneumograph,  etc.,"  Brain,  1907,  xxx.  153.  E.  B.  Titchener,  The  Psychology 
of  Feeling  and  Attention,  New  York,  1908. 


22 


LABORATORY  EXERCISES 
FOREWORD 

MOST  of  the  following  exercises  require  the  co-operation  of  two 
students,  one  of  whom,  the  "experimenter,"  conducts  the  experi- 
ment upon  the  other,  who  is  called  the  "  subject."  It  will  be  found 
advisable  that  the  same  pairs  of  students  should  work  together  through 
a  fairly  long  series  of  exercises,  rather  than  that  the  pairs  be  changed 
at  each  meeting  of  the  class.  Experimenter  and  subject  thus  get 
to  work  together  with  the  desired  smoothness.  On  the  other  hand, 
an  occasional  change  has  the  great  advantage  of  impressing  on  the 
students  the  enormous  individual  differences  met  with  in  different 
subjects. 

The  members  of  each  pair  should  reverse  places  in  successive  ex- 
periments. It  is  important  that  at  the  outset  students  should  realise 
the  equal  importance  of  the  roles  of  subject  and  experimenter.  A 
detailed  record  of  the  mode  in  which  an  experiment  has  been  carried 
out,  and  of  the  data  which  have  been  obtained  by  the  experimenter,  is 
psychologically  almost  valueless  unless  it  be  accompanied  by  a  corre- 
spondingly ample  introspective  record  on  the  part  of  the  subject. 
The  subject  should  be  encouraged  to  take  note  of  every  introspective 
detail,  however  trifling  it  appear.  In  some  experiments  it  will  be 
found  convenient  for  the  subject  to  dictate  the  results  of  his  introspec- 
tion to  the  experimenter,  who  writes  them  down.  But  better  results 
will  usually  be  gained  when  the  experiment  permits  of  the  subject 
himself  recording  these  data,  either  as  they  occur,  or  shortly  after, 
during  a  break  in  the  experiment. 

Every  experiment  should  be  carefully  written  out  at  home  from 
the  data  obtained  in  the  laboratory.  The  notebooks  of  experimenter 
and  subject  should  tally,  each  incorporating  the  other's  data,  so  as  to 
give  a  complete  account  of  the  same  experiment.  The  state  of  his 
notebook  affords  by  far  the  best  evidence  of  the  student's  diligence 
and  ability. 

It  need  scarcely  be  said  that  the  success  of  an  experiment  depends 
much  on  the  harmonious  working  of  subject  and  experimenter.  The 

339 


340  EXPERIMENTAL  PSYCHOLOGY 

sooner  an  uncongenial   pair  of  workers  dissolves  partnership,   the 
better. 

Those  who  desire  a  more  elaborate  account  of  laboratory  practice 
should  consult  the  Experimental  Psychology  (4  vols.,  New  York, 
1901-5)  of  Professor  E.  B.  Titchener.  No  writer  on  the  subject 
can  escape  laying  himself  under  a  debt  of  obligation  to  this  work. 
The  Course  of  Experimental  Psychology,  by  Professor  E.  C.  Sanford 
(Boston,  1897-8),  although  covering  less  ground,  can  also  be  warmly 
recommended.  But  many  of  the  experiments  following  are  not  given 
in  either  of  these  books. 


EXEECISES  ON  CHAPTEE  II 

Cutaneous  and  Visceral  Sensations 

TEMPERATURE  SPOTS. 

Bxp.  1.  The  student  should  first  gain  a  general  idea  of  the 
character  of  the  sensations  produced  by  the  stimulation  of  cold  and 
heat  spots.  Let  him  successively  and  lightly  touch  neighbouring 
points  of  the  skin  of  his  hand  or  arm  with  a  pencil  or  other  cold 
round-pointed  object,  and  observe  how  sensations  of  cold  flash  out 
from  time  to  time.  A  round-pointed  metal  object,  warmed  in  water 
or  in  the  flame  to  a  not  uncomfortable  temperature,  is  to  be 
similarly  moved  over  the  skin.  The  blunter  and  more  diffuse 
character  of  the  sensations  produced  by  heat  spots  will  be  at  once 
recognised. 

The  experimenter  now  proceeds  to  map  out  an  area  on  the  back  of 
the  subject's  hand,  measuring  about  20  by  10  sq.  mm.  He  then  takes 
one  of  a  number  of  metal  cylinders  which  have  been  immersed  in  a 
vessel  of  cold  water  standing  in  freezing  mixture. 

Having  dried  the  cylinder,  the  experimenter  begins  to  explore  the 
cold  spots  within  the  area  above  delimited.  The  subject  sits  with  his 
eyes  shut,  and  with  his  hand  loosely  closed  and  comfortably  supported  ; 
he  carefully  notes  his  experiences  and  records  them  periodically. 
The  exploration  must  be  done  in  a  systematic  manner,  the  cylinder 
being  methodically  applied  along  an  imaginary  series  of  lines,  1  mm. 
apart,  parallel  to  one  of  the  sides  of  the  square.  Along  these  lines  the 
cylinder  is  lightly  applied  to  consecutive  points  ;  it  is  always  moved 
in  the  same  direction,  and  where  the  subject  exclaims  that  he  feels  a 
pronounced  cold  sensation,  the  experimenter  marks  the  position  of  the 
cold  spot  in  coloured  ink  or  dye  upon  the  skin,  by  means  of  a  finely 


EXPERIMENTS  2-4  341 

pointed  brush.  The  experimenter  occasionally  puts  back  the  cylinder 
into  the  cold  water  and  takes  up  and  dries  a  fresh  one.  When  the 
area  has  been  explored,  point  by  point,  in  this  way,  the  experimenter 
draws  a  similar  area  on  two  pieces  of  tracing  paper,  which  he  applies 
to  the  skin,  marking  in  the  cold  spots  in  their  proper  position.  One 
of  these  two  records  is  for  the  subject's  notebook,  the  other  for  the 
experimenter's. 

Exp.  2.  The  heat  spots  are  to  be  sought  for  in  a  similar  way  over 
the  same  area,  after  the  previously  marked  dots  have  been  removed. 
These  spots  are  fewer  and  are  more  difficult  to  determine.  The  whole 
cylinder  must  be  warmed  in  hot  water  until  its  temperature  is  tolerable 
(about  48°  C.)  without  causing  unpleasantness.  In  the  course  of  the  ex- 
ploration the  cylinder  must  be  repeatedly  warmed, — an  inconvenience 
which  may  be  reduced  by  using  a  more  massive  metal  instrument,  e.g. 
a  finely  pointed  soldering  iron.  The  heat  spots  are  to  be  marked  and 
their  position  is  to  be  indicated,  in  dye  or  ink  of  another  colour,  upon 
the  tracing  paper  used  in  the  previous  experiment. 

Temperature  spots,  especially  some  of  them,  are  very  easily  fatigued. 
Hence  they  must  be  left  at  rest  before  the  area  is  re-investigated,  in 
order  to  confirm  previous  explorations. 

Exp.  3.  Sometimes  cold  sensations  occur  during  the  search  for 
heat  spots.  Having  selected  a  few  exceptionally  sensitive  cold  spots, 
the  experimenter  taps  one  of  them  lightly  with  a  very  small  round- 
pointed  object,  a  bristle  or  a  piece  of  wood.  Into  another  he  thrusts 
a  thin,  finely  pointed  needle.  Upon  a  third  he  tries  the  effect  of  a 
heated  (50°  C.)  point.  He  should  similarly  investigate  the  effect  of 
touch,  prick,  and  cold  upon  heat  spots. 

TOUCH  SPOTS. 

Exp.  4.  A  convenient  set  of  instruments  for  demonstrating  the 
existence  of  touch  spots  can  be  made  by  perpendicularly  mounting 
hairs  of  different  length  and  thickness,  each  at  the  end  of  a  match. 
The  pressure  exerted  by  a  hair  depends  chiefly  on  its  length  and 
thickness,  and  within  wide  limits  is  independent  of  the  extent  to 
which  it  is  bent.  This  pressure  may  be  measured  in  grams,  by 
applying  the  end  of  the  hair  to  one  of  the  scales  of  a  balance. 

The  experimenter  carefully  notes  the  points  of  emergence  of  all 
the  hairs  within  the  area  of  the  subject's  skin  already  delimited.  He 
marks  these  points  on  the  skin  in  ink  and  transfers  them  to  a  corre- 
sponding square  of  tracing  paper,  as  before.  A  magnifying-glass  should 


342  EXPERIMENTAL  PSYCHOLOGY 

be  used  to  detect  the  finer,  shorter,  or  fairer  hairs.  Then  the  hairs  are 
cut  off  by  fine  scissors  close  to  the  skin  surface,  and  the  dots  are  washed 
away.  (If  the  hairs  were  not  cut  off,  it  would  be  impossible  to  explore 
the  cutaneous  area  satisfactorily.  The  hairs  would  frequently  be 
touched  by  accident,  and  would  as  often  stimulate  the  underlying 
touch  spot.)  The  experimenter  selects  a  mounted  hair  which  provides 
a  stimulus  of  suitable  strength,  and  explores  the  area  systematically 
by  a  series  of  steady  touches  as  before,  the  long  axis  of  the  hair  being 
always  applied  perpendicularly  to  the  skin.  A  few  preliminary  experi- 
ments should  be  made  to  acquaint  the  subject  with  the  peculiar  sensation 
produced  by  a  touch  spot.  Each  touch  spot  is  to  be  marked  on  the  skin 
in  coloured  ink,  and  when  the  whole  area  has  been  explored,  the  dots 
are  to  be  transferred  to  the  paper  square  ;  their  relation  to  the  original 
position  of  the  hairs,  and  their  independence  of  the  position  of  the 
temperature  spots  being  noted.  The  subject  should  observe  the 
variations  in  character  of  the  sensation  produced  by  touch  spots  of 
different  sensitivity.  He  should  note  whether  any  other  sensations 
than  those  of  touch  are  simultaneously  or  subsequently  produced, 
and  he  should  observe  the  differences  in  accuracy  of  localisation  and 
in  the  apparent  depth  of  the  sensations  produced  by  the  temperature 
and  touch  spots. 

PAIN  SPOTS. 

Exp.  5.  Let  the  experimenter  lightly  touch  the  bent  knuckle  of 
a  finger  of  the  subject  with  a  finely  pointed  object,  e.g.  a  needle.  It 
is  easy  to  observe  that  at  certain  spots  the  distinctly  localised  touch 
sensation  is  followed,  after  an  obvious  interval,  by  a  more  radiating, 
ill-localised,  and  unpleasant  sensation  of  pain. 

Exp.  6.  The  experimenter  should  endeavour  to  discover  pain  spots 
in  a  small  part  of  the  hairless  area  already  used  in  the  previous  experi- 
ment. Care  must  be  taken  that  the  needle  never  pierces  the  skin.  For 
this  reason  it  is  preferable  to  use  pointed  horse  hairs  or  bristles, 
the  sensitivity  of  the  pain  spots  being  raised  by  a  thorough  soften- 
ing of  the  skin  with  soap  and  warm  water.  The  advantage  of 
pointed  hairs  lies  in  the  possibility  of  standardising  their  pressure 
(cf.  exp.  4). 

Exp.  7.  The  experimenter  selects  (a)  a  touch  spot  which  has  not 
a  pain  spot  in  its  immediate  neighbourhood,  and  (6)  a  pain  spot  which 
has  not  a  touch  spot  in  its  immediate  neighbourhood.  He  stimulates 
each  of  them,  and  observes  that  the  double  sensation  of  touch  and 
pain  obtained  above  is  no  longer  present. 


EXPERIMENTS  8-12  343 

Bxp.  8.  The  inside  of  the  cheek  is  explored  by  the  interrupted 
current  from  an.  induction  coil,  the  subject  satisfying  himself  as  to  the 
existence  of  a  painless  area. 

Exp.  9.  The  hand  is  dipped  into  water  at  50°  C.  The  initial 
sensation  of  temperature  preceding  that  of  pain,  is  observed. 

RELATION  OP  EXTENT  TO  INTENSITY  OF  THERMAL  STIMULATION. 

Exp.  1 0.  If  the  entire  hand  be  dipped  into  water  at  25°  C.,  and 
if  one  finger  of  the  other  hand  be  dipped  into  water  which  is  a  few 
degrees  higher  in  temperature,  it  will  be  observed  that  within  certain 
limits,  increase  of  the  extent  of  surface  stimulated  causes  increase  in 
the  intensity  of  the  temperature  sensation.  Similarly,  water  which 
is  not  uncomfortably  warm  to  a  small  area  of  the  body  becomes  in- 
tolerably painful  when  a  larger  surface  is  immersed. 

N.B. — It  will  be  observed  that  when  the  arm  is  immersed  in  water 
no  sensation  of  pressure  is  produced  save  at  the  line  of  emergence  of 
the  arm  from  water.  The  student  should  .consider  any  possible 
explanation  of  this  observation. 

TEMPERATURE  ADAPTATION. 

Exp.  11.  The  subject  places  a  finger  of  one  hand  in  water  at 
15°  C.,  and  the  corresponding  finger  of  the  other  in  water  at  35°  C. 
He  notes  the  gradual  changes  in  sensation,  and  after  a  few  minutes  he 
transfers  the  two  fingers  to  water  at  25°  C.,  observing  the  temperature 
sensations  in  each  of  the  fingers. 

N.B. — The  student  should  consider  how  it  is  that  there  is  a  difference 
between  the  temperature  sensations  afforded  by  touching  various 
objects,  solid  and  liquid,  rough  and  smooth,  about  the  room  ;  and 
why  it  is  that  the  same  room  feels  warmer  after  a  walk  on  a  windy 
day,  than  after  a  walk  on  a  windless  but  equally  cold  day. 

FATIGUE  OF  TEMPERATURE  SENSATIONS. 

Exp.  12.  The  subject  places  the  same  two  fingers  respectively  in 
water  at  45°  and  28°  C.,  the  latter  representing  approximately  the 
normal  temperature  of  the  skin.  After  about  fifteen  seconds  he  removes 
the  fingers  to  water  at  10°  C.  How  can  the  fact  be  explained  that  the 
coldness  of  the  latter  is  at  first  less  felt  by  the  finger  which  had  been 
previously  immersed  in  the  hotter  water  ?  The  experiment  may  be 
varied  by  transposing  the  two  vessels  of  water  at  10°  and  45°  C. 


344  EXPERIMENTAL  PSYCHOLOGY 

TEMPERATURE  AFTER-SENSATIONS. 

Bxp.  13.  The  subject  places  a  cold  coin  (about  5°  C.)  on  his  palm 
or  forehead  for  about  half  a  minute,  and  observes  the  after-sensation 
following  removal  of  the  coin.  Is  it  continuous  throughout  or  ever 
intermittent  ?  Does  it  differ  in  any  way  from  the  character  of  the 
cold  sensation?  Similarly,  the  subject  observes  the  after-sensation 
following  removal  of  a  warm  object.  He  should  consider  the  bearing 
of  the  two  experiments  on  Weber's  and  Hering's  theories.  He  should 
then  investigate  the  after-effects  of  a  stimulus  applied  for  two  minutes 
and  maintained  at  about  9°  C. 

TEMPERATURE  AND  WEIGHT  ILLUSION. 

Exp.  14.  The  subject  compares  the  weights  of  two  similar  coins 
placed  alternately  on  the  palm  (or  forehead),  the  one  coin  having  been 
previously  cooled,  the  other  having  been  warmed  approximately  to 
the  temperature  of  the  skin.  It  will  be  found  that  a  similar  illusion 
holds  for  objects  which  are  above  as  well  as  for  those  which  are  below 
the  skin  temperature. 


EXERCISES  ON  CHAPTERS  III  AND  IV 

Auditory  Sensations 

SOUND  CONDUCTION. 

Exp.  15.  The  foot  of  a  vibrating  tuning-fork,  c',  is  applied  to 
the  vertex  of  the  head  or  to  the  teeth.1  The  tone  reaches  the  ears  by 
bone  conduction.  It  is  only  when  the  membranes  and  ossicles  of  the 
middle  ear  are  defective  that  a  fork  (of  moderate  pitch)  is  audible  by 
bone  conduction,  when  it  is  inaudible  vid  the  outer  a,nd  middle  ear. 
This  should  be  verified  by  observing  that  after  the  tone  of  a  fork, 
applied  to  the  bone  behind  the  ear  (the  mastoid  process),  has  apparently 
ceased,  the  fork  can  again  be  heard  if  it  be  at  once  removed  and  held 
near  the  outer  ear. 

Exp.  16.  A  vibrating  tuning-fork  is  held  opposite  one  ear,  at  a 
distance  from  it  exceeding  the  distance  from  one  ear  to  the  other. 

1  The  student  should  make  it  a  rule  to  touch  the  prongs  of  tuning-forks  with 
the  uncovered  hand  as  rarely  as  possible.  The  warmth  of  the  hand  produces  a 
diminution  in  pitch,  and  its  moisture  makes  the  forks  very  liable  to  rust. 


EXPERIMENTS  17-20  345 

When  the  tone  has  become  too  feeble  to  be  audible,  the  fork  is  quickly 
brought  close  to  the  ear.  It  will  of  course  be  heard  again.  A  finger 
is  now  lightly  introduced  into  the  opposite  ear  hole.  The  effects  upon 
the  tone  that  are  produced  by  alternately  withdrawing  and  reintro- 
ducing  the  finger,  should  be  then  observed. 

N.B. — The  fork  is  first  held  at  a  distance  greater  than  that  between 
the  two  ears,  in  order,  so  far  as  possible,  to  meet  the  objection  that 
the  subsequent  effects  obtained  by  the  fmger  are  dependent  on  the 
passage  of  the  sound  to  the  more  distant  ear  through  the  external  air. 
To  account  for  the  effects,  the  following  facts  must  be  borne  in  mind. 
Bone  conduction  of  sounds  occurs  from  one  ear  to  the  other  (page  21). 
If  a  finger  is  lightly  placed  in  one  ear,  it  reflects  the  sound  waves 
which  are  travelling  from  within  outwards,  thus  preventing  their 
escape  and  intensifying  the  auditory  stimulation  of  that  ear.  A 
sound  is  localised  in  that  ear  which  receives  the  stronger  stimulus 
(page  288). 

EESONANCE. 

Bxp.  17.  The  student  should  familiarise  himself  with  the 
phenomena  of  resonance,  by  using  a  series  of  tuning-forks  and  re- 
sonators. He  should  identify  the  resonator  which  is  attuned  to 
vibrate  to  any  particular  fork.  Having  by  means  of  a  movable  clamp, 
slightly  mistimed  a  fork,  he  should  note  the  corresponding  alterations 
in  reinforcement  by  the  resonator. 

Bxp.  18.  If  the  loud  pedal  of  a  pianoforte  be  pressed  down  (in 
order  that  the  strings  may  be  free  to  vibrate),  the  resonant  effect  of 
singing  tones  before  it  may  be  readily  observed. 

Bxp.  19.  The  external  meatus  itself  behaves  as  a  resonator.  One 
tone  more  than  any  other  in  the  neighbourhood  of/iv  will  be  found  to 
have  a  piercing  character.  The  pitch  of  this  tone  should  be  deter- 
mined by  means  of  a  small  whistle.  The  resonant  effect  of  the  meatus, 
thus  discovered,  may  be  changed  by  inserting  a  piece  of  rubber  tubing 
about  half  an  inch  long  into  each  ear. 

NOISE. 

Bxp.  20.  As  opportunity  arises,  the  student  should  examine  the 
character  of  different  noises  introspectively,  noting  their  varying  dis- 
similarity from  tones,  and  endeavouring  to  detect  their  pitch.  He 
should  depress  a  great  number  of  adjacent  keys  on  the  pianoforte 
simultaneously, — or  still  better,  sound  numerous  adjacent  tones  on 


346 


EXPERIMENAL  PSYCHOLOGY 


a  Tonmesser  (fig.  25), — and  observe  the  noisy  character  of  the  resulting 
experience.  This  instrument,  once  made  by  the  firm  of  Appunn,  and 
often  called  after  the  original  maker,  contains  a  series  of  small  metal 
tongues  M  which  are  enclosed  in  a  case  and  blown  by  a  bellows.  The 
tongues  are  so  attuned  that  the  several  tones  they  emit  differ  only 
slightly  (by  one,  two,  or  four  vibrations)  from  one  another.  Each 
tongue  can  be  sounded  or  silenced  by  pulling  out  or  pushing  in  the 
stop  S  attached  to  it.  "When  in  use,  the  case  is  closed  and  mounted 
on  a  table  containing  the  bellows,  from  which  air  enters  the  case 
atT. 

The  student  should  notice  the  effects  of  coughing  or  "  clearing  the 


FIG.  25. 

throat"  before  a  pianoforte.  The  effect  of  practice  upon  the  detection 
of  pitch  in  noises  is  well  shown  by  successively  dropping  wooden 
pencils  of  different  length  on  to  a  wooden  table. 


TIMBRE. 

Exp.  21.  Tones  of  identical  pitch  should  be  produced  from 
various  instruments  (strings,  whistles,  forks,  metal  tongues,  sirens, 
etc.)  in  order  that  the  inherent  differences  of  timbre  may  be  closely 
noticed.  It  is  easy  for  the  student  to  analyse  the  overtones  by  using 
the  appropriate  resonators.  The  pitch  of  the  fundamental  tone  being 
known,  he  should  calculate  the  pitch  of  the  harmonic  series  of  its 
overtones. 


EXPERIMENTS  22-26  347 

Exp.  22.  The  experimenter  plucks  the  string  of  a  monochorcl,  and 
as  its  tone  is  dying  away  he  touches  it  lightly  with  a  small  brush  or 
feather  at  either  of  the  points  which  trisects  its  length.  He  repeats 
this  several  times,  the  subject  always  listening  carefully  to  the  pitch 
of  the  tone  produced  by  the  brush.  The  experimenter  touches  the 
string  with  the  brush  each  time  more  lightly  than  before.  Ultimately, 
when  the  brush  is  not  used  at  all  and  the  vibrations  of  the  string  are 
allowed  to  die  away  undisturbed,  the  subject  will  distinctly  recognise 
that  overtone  of  the  string,  which  has  the  same  pitch  as  the  tone 
produced  by  the  brush.  A  similar  procedure  should  be  adopted  in 
order  to  detect  other  overtones  of  the  string. 

N.B. — No  special  musical  ability  or  previous  training  is  required  to 
observe  many  of  these  overtones  successfully.  After  a  little  practice 
they  may  be  detected  in  a  prolonged  note  of  the  pianoforte.  An 
attempt  should  be  made  to  determine  by  introspection  the  effect  of 
such  analyses  upon  the  character  of  the  whole  tonal  experience. 

Exp.  23.  A  fork  and  its  octave  fork,  mounted  on  their  respective 
resonance  boxes,  are  simultaneously  sounded.  As  the  vibrations 
lessen  the  student  notes  the  difference  in  timbre  (comparable  to  a 
change  of  vowel  in  the  voice)  produced  by  stopping  the  vibrations  of 
the  higher  fork. 

AFTER -SENSATIONS. 

Exp.  24.  The  experimenter  suddenly  stops  the  vibrations  of  a 
tuning-fork  while  it  is  held  before  the  ear  of  the  subject.  The  latter 
carefully  observes  whether  he  can  recognise  any  after-sensations.  If 
they  are  present,  he  endeavours  to  record  their  number,  duration, 
character,  and  the  interval  elapsing  between  the  removal  of  the 
stimulus  and  their  first  appearance. 

TONE  CHARACTER. 

Exp.  25.  The  student  should  note  the  character  of  tones  of  different 
pitch. 

THE  INTENSITY  OF  SIMULTANEOUS  TONES. 

Exp.  26.  A  high  fork  and  a  low  fork  are  simultaneously  sounded 
upon  their  resonance  cases,  and  their  vibrations  are  allowed  to  die 
away  until  the  high  fork  can  no  longer  be  heard.  If  now  the  vibra- 
tions of  the  low  fork  be  stopped,  the  high  fork  will  at  once  be  heard 
again.  On  the  other  hand,  if  the  sounds  are  allowed  to  continue 
until  the  low  fork  is  no  longer  heard,  the  audibility  of  the  latter  will 
not  be  revived  by  stopping  the  higher  fork.  That  is  to  say,  a  low 


348  EXPERIMENTAL  PSYCHOLOGY 

tone  will  obliterate  a  weak  high  tone  far  more  completely  than  a  high 
tone  will  obliterate  a  weak  low  tone. 

THE  UPPER  LIMIT  OF  PITCH. 

Exp.  27.  The  upper  limit  of  hearing  is  here  determined  by 
means  of  a  Galton's  whistle. 

In  blowing  the  instrument,  care  must  be  taken  that  the  wind 
pressure  employed  be,  as  nearly  as  possible,  uniform.  The  subject 
sits  side \vays  at  about  a  metre's  distance  from  the  experimenter.  The 
latter  takes  the  whistle  and  sets  it  so  as  to  produce  a  relatively  low 
tone.  The  whistle  length  is  gradually  shortened  after  each  note  is 
produced,  until  a  point  is  reached  when  the  subject  can  hear  no  tone, 
but  only  the  puff  of  the  windblast.  The  experimenter  records  the 
length  of  the  whistle  at  this  point,  shortens  it  yet  a  little,  and  then 
commences  a  fresh  series  of  observations,  gradually  lengthening  the 
whistle  until  the  subject  just  recognises  the  presence  of  a  tone.  Again 
the  experimenter  records  the  whistle  length.  Five  pairs  of  such 
records  should  be  taken,  and  the  mean  of  the  ten  estimations  be 
determined. 

BINAURAL  DIFFERENCES  IN  PITCH. 

Exp.  28.  The  subject  holds  two  tuning-forks  of  identical  pitch, 
one  in  each  hand,  ready  for  the  experimenter  to  strike  them.  The 
subject  then  lifts  them  several  times  alternately,  the  right-hand  fork 
to  the  right  ear,  the  left-hand  fork  to  the  left  ear,  and  will  perhaps 
ultimately  decide  that  the  two  forks  appear  to  be  of  different  pitch. 
The  experimenter  applies  a  light  clamp  or  a  small  lump  of  wax  to 
the  prongs  of  the  apparently  higher- sounding  fork,  and  hands  them 
to  the  observer.  They  are  struck  by  the  experimenter,  and  the 
observer  again  compares  their  tones.  The  clamp  is  raised  or  lowered 
until  the  tones  appear  identical. 

N.B. — In  seeking  to  account  for  the  differences  (if  any)  obtained, 
the  student  must  bear  in  mind  the  various  circumstances  (pages  31,  32) 
in  which  an  illusory  change  of  pitch  is  possible. 

BEAT-COUNTING. 

Exp.  29.  The  student  simultaneously  sounds  the  two  forks,  which 
have  been  brought  to  apparently  identical  pitch  in  the  previous 
experiment.  He  proceeds  to  count  the  beats,  aided,  if  necessary,  by 
holding  the  two  forks  over  a  single  resonator.  Provided  that  the 
frequency  of  the  beats  do  not  exceed  five  per  second,  they  may  be 
counted  directly.  Beyond  this  limit,  an  intermediate  fork  must  be 


EXPERIMENTS  30,  31 


349 


introduced,  which  is  to  be  sounded  first  with  one  and  then  with  the 
other  fork,  the  beats  being  counted 
in  each  case.     The  beats  are  to  be  M, 

counted  singly,  or  in  pairs  or  in 
fours.  Ten  counts  in  all  should 
be  made,  by  different  methods  of 
reckoning,  if  possible.  The  count- 
ing must  be  begun  when  the  posi- 
tion of  the  hand  of  the  watch, 
as  it  lies  exactly  over  a  second's 
mark,  coincides  with  a  beat.  The 
student  must  start  thus  :  0,  1,  2, 
3,  4  ;  he  must  remember  to  deduct 
one  from  the  result  if  he  start 
from  unity.  He  should  count  for 
fifteen  seconds,  and  then  calculate 
the  mean  number  of  beats  counted 
per  second.  Then  he  can  express 
in  terms  of  a  fraction  of  a  tone 
the  difference  between  the  two 
ears  in  the  determinations  of  pitch. 

THE  FEATURES  OF  BEATS. 

Bxp.  30.  The  four  stages,  al- 
luded to  on  pp.  38,  39,  should  be 
observed  as  the  frequency  of  beats  Flo<  26.-This  instrument  consists  essen 


is  gradually  increased.  Two  tun- 
ing forks  giving  c  may  be  em- 
ployed, the  pitch  of  one  of  which 
is  gradually  lowered  by  adjustable 
clamps.  Stern's  Tone  Variators, 
however  (fig.  26),  provide  a  far 
more  convenient  apparatus. 

Bxp.  31.  Beats  obtained  from 
different  tone  regions  are  to  be 
compared. 

A  comparison  is  made,  if  pos- 
sible, between  the  successive  pairs 
of  tones  C0  G0,  G0  c°,  cc  e°,  e°  g°,  c 
rf',  b'  c",  each  of  which  gives  about  33  beats  per  second.     The  student 
should  note  the  differences  in  roughness  according  to  the  tone  region. 


tially  of  a  bottle  B  which  is  blown 
at  its  mouth  M.  The  column  of  air 
within  the  bottle  is  shortened  or 
lengthened  by  the  upward  or  down- 
ward movement  of  the  piston  P. 
The  piston  is  moved  by  the  rotation 
of  a  peculiarly  shaped  metal  disc, 
the  edge  of  which  is  just  visible  at 
V.  The  rotation  of  this  disc  (or 
"variator")  is  dependent  on  move- 
ment of  the  two  connected  graduated 
wheels  Y  and  X,  the  latter  of  which, 
when  manipulated  by  the  experi- 
menter, thus  effects  minute  or  rela- 
tively gross  changes  in  the  pitch  of 
the  note  emitted  by  the  bottle. 


350  EXPERIMENTAL  PSYCHOLOGY 

THE  INTERTONE. 

Bxp.  32.  The  pitch  of  the  beating  tone  (the  intertone)  is  care- 
fully noted  as  the  interval  between  two  nearly  identical  tones  is 
increased. 

DIFFERENCE  TONES. 

Bxp.  33.  The  student  takes  two  Quincke's  tubes  (fig.  27)  of 
high  pitch,  e.g.  civ,  eiv,  giving  an  interval  of  a  major  third.  He 
sounds  them  alternately  in  increasingly  rapid  succession,  paying 
careful  attention  to  the  pitch  of  each  note.  Finally,  while  the  lower 
tone  is  sounding,  he  introduces  the  higher.  After  a  little  practice 
he  will  be  able  to  observe  the  deep  difference 
tone  and  its  peculiar  localisation. 

Exp.  34.  The  student  simultaneously 
blows  (if  possible  with  a  blast  of  regulated  con- 
stant pressure)  two  ordinary  piston  whistles, 
— better  still,  two  of  Stern's  Tone  Variators 
(fig.  26), — starting  from  unison  and  gradually 
raising  the  pitch  of  one  of  them.  At  first 
only  beats  are  heard ;  next,  possibly  the 
difference  tone  of  the  second  order ;  later, 
the  very  low  difference  tone  of  the  first  order 
appears,  which  rises  in  pitch  as  the  interval 
between  the  primary  tones  increases. 

FIG.  27.  Bxp.  35.  The  student  takes  two  tuning- 

forks  of  known  pitch,  which  give  an  audible 

difference  tone  of  the  first  order.  He  sounds  these  on  their  resonance 
boxes  and  compares  the  pitch  of  the  difference  tone  with  that  of  a 
suitable  simultaneously  sounding  fork,  the  tone  of  which  can  be 
varied  by  means  of  an  adjustable  clamp.  Beats  will  be  heard  as  the 
pitch  of  the  latter  fork  is  brought  near  to  that  of  the  difference 
tone,  the  beats  becoming  slower  with  diminishing  difference  of  pitch 
and  disappearing  when  absolute  unison  is  reached.  In  order  to 
determine  the  pitch  of  the  difference  tone,  the  pitch  of  the  tone  given 
by  the  fork  thus  clamped  must  be  found  by  making  it  beat  with 
another  fork  of  known  pitch. 

Exp.  36.  If  two  Quincke's  tubes  (with  corks  removed)  which 
lie  a  major  seventh  apart  (8  : 15)  be  sounded  together,  careful  observa- 
tion will  reveal  the  presence  of  the  deep  difference  tone  of  the  second 
order  lying  three  octaves  below  the  lower  tone. 


EXPERIMENTS  37-40  351 

THE  RELATIONS  OF  TONES. 

Exp.  37.  The  student  should  familiarise  himself  on  the  piano- 
forte with  various  intervals  within  and  beyond  the  octave,  playing 
the  tones  of  each  interval  both  successively  and  simultaneously,  and 
using  the  same  tonic  (page  29)  throughout.  He  should  note  the 
intimate  relation  of  a  tone  to  its  octave,  and  the  differences  between 
the  various  consonant  and  dissonant  intervals. 

Exp.  38.  The  different  degrees  in  which  fusion  is  manifested 
may  be  easily  studied  if  the  experimenter  sound  sometimes  two  tones, 
and  at  other  times  only  a  single  tone,  the  subject  deciding  whether 
one  or  two  tones  are  present.  The  number  of  correct  answers  given 
by  an  unmusical  subject  varies  with  the  degree  of  fusion.  The  ease  of 
analysis  of  a  given  interval  may  be  also  examined  when  the  latter  is 
increased  by  one  or  more  octaves. 


EXEECISES  ON  CHAPTEK  V 

Labyrinthine  and  Motor  Sensations 

Labyrinthine  Sensations. 

The  experimental  results'  recorded  on  pages  65-66  are  readily 
capable  of  verification.  A  turntable  is  required  for  passive  rotation. 

Motor  Sensations. 
THE  DISTINCTION  BETWEEN  CUTANEOUS  AND  MOTOR  SENSATIONS. 

Exp.  39.  The  motor  sensations,  which  occur  during  the  move- 
ments produced  by  faradic  stimulation  of  appropriate  points  an  the 
forearm,  are  observed. 

PASSIVE  MOVEMENT. 

Exp.  40.  The  subject's  arm  is  bared  and  supported  in  a  comfort- 
able resting  position.  His  eyes  are  closed.  The  experimenter  places 
011  the  forearm  a  weight ;  the  base  of  which  is  covered  with  a  pad  of 
blotting  paper,  in  order  to  reduce  the  conduction  of  heat  from  the 
skin.  He  leaves  it  there  for  several  seconds.  The  subject  records  the 
various  experiences  which  he  obtains  during  and  after  the  application 
of  the  weight.  After  allowing  a  sufficient  interval  of  rest,  the  experi- 


352  EXPERIMENTAL  PSYCHOLOGY 

menter  next  applies  a  much  heavier  or  lighter  weight  to  the  subject's 
arm,  and  a  similar  introspective  record  is  obtained. 

Bxp.  41.  The  same  experiment  is  performed  while  the  skin  is 
being  rendered  anaesthetic  by  spraying  it  with  ether.  The  skin  is 

sprayed  for  about  half  a  minute, 
and  then  the  weight  is  placed 
upon  it  for  a  few  seconds. 
Thereupon  the  weight  is  re- 
moved and  the  spraying  is  re- 
commenced. This  alternation 
is  continued  until  the  skin  is 
quite  anaesthetic.  Careful  in- 
trospective records — and  these 
are  easily  procurable  after  a 
little  training  —  will  yield  in- 
teresting points  of  comparison 
FIG.  28.  with  the  records  obtained  in  the 

previous  experiment. 

Exp.  42.  The  effects  of  passive  movement  may  be  roughly  studied 
by  means  of  the  apparatus  illustrated  in  figure  28.  The  subject's 
forearm  is  laid  on  the  hinged  board,  which  is  moved  by  the  experi- 
menter pressing  down  the  counterweight.  It  is  easy  to  observe  that 
sensations  of  tension  precede  those  of  movement,  and  that  the  latter 
may  occur  although  the  subject  is  unable  to  determine  the  direction 
of  movement. 

EXPERIENCE  OF  RESISTANCE. 

Exp.  43.  A  finger  of  the  subject's  hand  is  fitted  with  a  band,  to 
which  is  attached  a  thread  carrying  a  weight.  The  subject,  blind- 
folded, holds  his  arm  horizontally  away  from  the  body,  and  proceeds 
slowly  to  lower  the  arm.  Meanwhile  the  experimenter  suddenly  and 
noiselessly  removes  the  weight.  The  subject  will  note  the  sensation 
of  resistance  and  the  tendency  to  upward  movement  of  the  limb.  He 
should  carefully  record  these  and  other  experiences  occurring  during 
the  experiment,  and  he  should  endeavour  to  interpret  them  in  the 
light  of  his  knowledge  of  motor  sensations. 

ESTIMATION  OF  LIMINAL  AND  SUPRALIMINAL  MOVEMENTS. 

Exp.  44.  For  accurately  studying  the  limen  or  threshold  of 
just  perceptible  active  or  passive  movement  in  different  joints, 


EXPERIMENT  44  353 

apparatus  must  be  so  contrived  as  to  permit  of  movement  only 
in  the  joint  which  is  under  consideration.  Thus,  in  studying  the 
movement  of  a  finger  joint,  the  palm  and  the  remaining  fingers  are 
steadied  in  a  plaster  mould,  and  are  kept  at  rest  by  the  pressure  of 
cushions,  while  the  other  joints  of  the  finger  are  securely  fixed. 
Under  such  conditions,  the  finger  may  be  passively  moved  by  means 
of  a  finger-cap  fitting  on  to  the  tip  of  the  finger,  a  thread  passing  from 
this  cap  over  a  pulley  and  terminating  in  a  (variable)  weight.  A  lever 
may  be  attached  to  the  thread  and  brought  to  bear  on  a  travelling 
smoked  surface,  whereby  the  extent  and  speed  of  the  movement  of 
the  finger,  whether  it  be  moved  actively  or  passively,  may  be  re- 
corded. 

Such  an  experiment  is  too  complicated  for  class  work.  On  the 
other  hand,  the  apparatus  for  studying  appreciation  of  differences  in 
extent  of  movement  may  be  of  a  quite  simple  character.  A  graduated 
ruler,  to  which  a  rider  and  a  stop  can  be  fixed,  will  indeed  suffice. 
A  more  convenient  form  of  the  apparatus  (fig.  29)  consists  of  a  little 
trolley  T  made  to  receive  the  finger  at  F.  This  trolley  travels  easily  along 
rails  laid  upon  a  graduated  board,  which  can  be  placed  in  any  position. 
The  subject,  standing  or  seated  conveniently  before  the  apparatus 
with  his  eyes  closed,  places  his  finger  in  the  trolley,  and  his  arm 
executes  actively  or  passively  the  desired  movement.  A  string, 
pulley  P,  and  weight  may  be  attached  to  the  trolley,  so  as  to 
increase  or  decrease  the  necessary  force  of  movement.  The  excur- 
sions of  the  trolley  can  be  limited  in  either  direction  by  the  movable 
stops  S'  S". 

With  such  an  apparatus,  the  arm  of  a  blindfold  subject  may  be 
actively  or  passively  moved  to  a  known  extent,  from  a  known  posi- 
tion, in  a  known  direction,  with  a  known  speed.  And  after  a  known 
interval  of  time  the  subject,  still  blindfold,  may  be  asked  to  make  an 
apparently  equally  extensive  movement,  from  the  same,  or  from 
a  different,  original  position  of  the  arm,  with  the  same  or  with  the 
opposite  arm,  in  the  same  or  in  a  different  direction,  with  the  same 
or  with  a  different  speed :  and  the  accuracy  of  his  estimate  is 
recorded.  Or,  after  a  known  interval,  the  subject's  arm  may  be 
passively  moved  in  a  pre-determined  manner,  and  his  judgment  in 
comparing  the  extent  of  the  two  movements  recorded.  Or  again, 
two  such  trolleys  may  be  prepared,  so  that  the  effects  and  the  com- 
parison of  simultaneous  movements  of  the  two  arms  may  be  investi- 
gated. 

It  is  obvious  that  a  great  variety  of  psychological  experiments 
can  be  performed  in  this  simple  way.  But  the  conditions  laid 
down  in  Chapter  XV.  must  be  scrupulously  observed.  The  student 

23 


354 


EXPERIMENTAL  PSYCHOLOGY 


is  therefore  advised  to  postpone  working  for  the  present  with  this 
apparatus. 

One  defect  of  such  an  instrument  is  that  it  provides  no  means  of 
insuring  a  constancy  of  the  share  which  the  different  joints  (e.g.  the 


FIG.  29. 


finger,  the  wrist,  the  elbow,  and  the  shoulder)  take  in  each  total 
movement.  Another  objection  is  that  it  does  not  eliminate  the 
complicating  effects  of  sensations  of  cutaneous  and  deep  pressure. 
With  practice,  however,  a  sufficiently  uniform  movement  is  attained. 
And  it  has  the  advantage  of  being  a  movement  comparable  to  the 
movements  made  in  everyday  life. 


EXPERIMENTS  45,  46 


355 


EXEECISES  ON  CHAPTERS  VI  AND  VII 

Visual  Sensations 
THE  CHARACTERS  OP  COLOURLESS  AND  COLOUR  SENSATIONS. 

Bxp.  45.  The  series  of  colourless  sensations  obtained  by  varying 
the  proportions  of  black  and  white  upon  the  colour  wheel  (fig.  30) 
should  be  carefully  observed.1 

Bxp.  46.  The  saturation  of  a  colour  is  to  be  changed  by  mixing 
with  it  on  the  colour  wheel  varying  amounts  of  white.  The 


FIG.  30.— In  the  colour  wheel  here  illustrated,  the  paper  discs  may  be 
mounted  both  at  A'  and  at  A",— an  arrangement  which  allows  of  the 
simultaneous  exhibition  and  matching  of  two  colour  mixtures.  In  the 
more  usual,  simpler  form  of  the  colour  wheel,  the  papers  can  be 
mounted  on  one  axis  only. 

results  of  mixing  colours  with  black  should  be  also  observed,  and 
subsequently  reconsidered  in  their  relation  to  the  theories  of  colour 
vision. 

1  When  papers  are  mounted  on  the  colour  wheel,  an  uncut  paper  disc  of  the 
same  diameter  nrast  always  be  placed  behind  them.  Before  the  nut  is  screwed 
on,  a  minute  disc  of  thick  paper,  having  the  same  diameter  as  that  of  the  nut,  is 
interposed,  in  order  to  prevent  the  pressure  of  the  latter  from  marking  and 
wearing  out  the  papers.  Care  must  be  taken  to  arrange  the  papers  and  to  turn 
the  colour  wheel  in  a  direction  so  that  the  free  edges  of  the  papers  lie  flat  during 
rotation  ;  otherwise,  by  flying  up,  they  will  become  torn. 


356  EXPERIMENTAL  PSYCHOLOGY 

Exp.  47.  The  varying  brightness  of  differently  coloured  papers 
should  be  noted.  The  degrees  of  brightness  are  to  be  determined  by 
comparing  them  with  papers  of  the  colourless  series.  If  the  black- 
white  values  of  the  latter  be  known,  the  brightness  of  the  various 
colours  may  be  expressed  in  terms  of  these  values;  thus 
Y  =  178°  W+182°BK. 

The  student  should  be  in  a  position  to  answer  for  himself  the 
question,  How  may  the  saturation  of  a  colour  be  changed  upon 
the  colour  wheel,  while  its  brightness  remains  unaltered  ? 

Exp.  48.  The  observer  sets  up  two  squares  of  equally  white 
papers  at  different  distances  from  a  window,  one  behind  the  other, 
so  that  when  he  looks  at  them  with  one  eye  directed  through  an 
open  tube  he  obtains  a  circular  plane  field,  filled  half  with  the  one 
and  half  with  the  other  paper.  Having  set  the  papers  at  a  distance 
from  one  another,  which  is  just  sufficient  to  obtain  distinctly  different 
degrees  of  brightness  when  they  thus  appear  to  be  situated  in  the 
same  plane,  the  observer  removes  the  tube  and  compares  the  bright- 
ness of  the  papers  in  ordinary  vision. 

Exp.  49.  Three  spectral  colours  are  to  be  chosen,  so  that  when 
mixed  in  appropriate  proportions  on  the  colour  wheel,  they  give 
rise  to  a  colourless  sensation. 

The  various  hues,  spectral  and  extra-spectral,  obtained  by  mixing 
these  colours  in  other  proportions  should  be  also  noted. 

COLOUR  MIXTURES. 

Exp.  50.  The  observer  finds  the  complementary  colour  to  any 
colour,  so  that  the  two  coloured  papers,  when  turned  simultaneously 
upon  the  same  colour  wheel,  may  give  rise  to  a  colourless  sensation. 

N.B. — It  is  at  first  puzzling  to  find  that  the  blue  and  yellow 
papers,  placed  together  on  the  colour  wheel,  produce  a  sensation  dif- 
ferent from  that  obtained  by  mixing  blue  and  yellow  pigments ;  but 
the  explanation  is  easy.  The  papers  are  specially  selected  for  their 
purity  of  colour,  while  ordinary  blue  and  yellow  pigments  contain 
green,  which  becomes  evident  when  the  blue  and  yellow  neutralise 
one  another. 

Exp.  51.  Two  small  coloured  paper  squares  of  like  dimensions 
are  placed  upon  a  black  velvet  ground.  Between  them  is  set  a 
vertical  piece  of  glass;  and  the  head  of  the  observer  is  so  placed 
that  the  one  colour,  seen  by  light  transmitted  through  the  glass,  and 


EXPERIMENTS  52,  53 


357 


FIG.  31. 


the  other,  seen  by  light  reflected  from  the  glass,  fall  on  the  same 
area  of  the  retina  (fig.  31).  The  colour  sensation  thus  produced 
should  be  compared  with  that  ob- 
tained  by  the  rotation  of  the  same 
two  colours  on  the  colour  wheel. 

PERIPHERAL  COLOUR  VISION. 

Bxp.  52.  The  Perimeter  en- 
ables the  subject  to  demonstrate 
the  regions  of  total  and  partial 
colour  "blindness"  towards  the 
periphery  of  the  retina,  the  ex- 
perimenter moving  towards  the 

centre  a  small  square  of  coloured  paper  along  the  free  arm  of  the 
instrument.  The  experimenter  selects  such  colours  as  orange,  blue- 
green,  or  purple.  He  notes  the  points  at  which  the  colour  just  begins 
to  be  visible  to  the  subject  as  a  colourless  spot.  Within  this  zone  he 
maps  out  another,  in  which  the  colour  acquires  a  yellowish  or  bluish 
tinge.  Finally,  he  defines  the  innermost  area,  in  which  the  colour 
acquires  a  reddish  or  greenish  tinge. 

Under  ordinary  conditions  the  intermediate  zone  for  yellow  and 
that  for  blue  vision  are  rarely  quite  coincident.  But  when  care  is 
taken  that  the  two  colours  have  equal  chromatic  and  achromatic  (i.e. 
brightness)  values,  the  limits  are  identical.  These  conditions  are 
satisfied  when  the  two  complementary  coloured  papers  are  so  chosen 
that  they  require  to  be  mixed  in  equal  proportions  to  give  rise  to  a 
colourless  sensation,  and  that  they  are  alike  in  size,  brightness,  and 
illumination. 

An  attempt  should  be  made  to  find  four  colours  which,  as  they 
are  passed  from  the  periphery  over  the  retina,  give  rise  (at  first  of 
course  to  colourless,  and  next)  to  colour  sensations,  the  hue  of  which 
subsequently  remains  unaltered  as  the  stimulus  is  moved  still  farther 
towards  the  fovea. 

NEGATIVE  IMAGES. 

Exp.  53.  A  black  square  on  a  white  ground  is  fixated  for  a  few 
seconds.  The  eyes  are  then  closed,  or  they  are  directed  to  a  large, 
uniform  grey  or  white  surface.  The  observer  should  note  (i.)  the 
degree  of  brightness  of  the  after-image  of  the  square  and  of  its 
background,  (ii.)  the  series  of  reappearances  of  the  after-image,  (iii.) 
the  degree  of  distinctness  of  the  margins  of  the  after-image  of  the 
square. 


358  EXPERIMENTAL  PSYCHOLOGY 

Exp.  54.  He  should  next  fixate  a  white  square  on  a  black 
ground,  and  obtain  the  after-image  on  a  grey,  white,  or  black  surface, 
making  observations  as  before.  He  should  note  whether  the  margins 
of  the  after-image  of  the  square  are  as  distinct  as  in  the  previous 
experiment,  and  whether  the  duration  of  original  fixation  affects  the 
after-image  and  the  breadth  or  brightness  of  the  halo  (sometimes 
called  the  "  corona  ")  which  surrounds  it. 

Exp.  55.  Similar  after-images  should  be  obtained  from  coloured 
squares  upon  colourless  grounds.  Coloured  after-images  should  also 
be  projected  on  squares  of  complementary  or  other  colours.  The 
after-image  of  a  white  square  on  a  black  ground  should  be  projected 
on  to  an  orange  ground.  The  resulting  experience  should  be  re- 
membered in  considering  later  the  nature  of  black. 

Exp.  56.  The  blue -green  after-image  of  a  small  red  patch, 
fixated  on  a  white  background,  is  projected  on  to  a  black  velvet 
background,  and  the  brightness  and  saturation  of  this  after-image  is 
compared  with  that  of  a  small  blue-green  patch  lying  on  the  black 
velvet  a  few  millimetres  away  from  the  point  of  projection  of  the 
after-image.  The  student  should  consider  later  whether  the  results 
of  the  comparison  are  favourable  to  Helmholtz's  theory  of  the  cause 
of  after-images. 

SIMULTANEOUS  CONTRAST. 

Exp.  57.  A  disc  containing  a  middle  zone  of  black  and  white, 
surrounded  by  a  given  colour,  is  rotated  on  the  colour  wheel.  The 
contrast  colour  is  most  marked  when  the  coloured  and  colourless 
surfaces  are  of  equal  brightness. 

Exp.  58.  A  grey  paper  is  successively  placed  on  different 
coloured  backgrounds,  which  are  equal  to  it  in  brightness.  If  the 
contrast  effect  is  not  apparent,  it  is  immediately  forthcoming  when 
the  grey  surface  and  its  adjoining  background  is  covered  with  tissue 
paper.  The  contrast  effect  is  reduced  if  a  pencil  line  be  now  drawn 
on  the  tissue  paper,  corresponding  to  the  margins  of  the  underlying 
grey  paper. 

Exp.  59.  The  observer  illuminates  a  white  opal  surface 
simultaneously  with  coloured  light  and  with  colourless  light  from 
two  different  sources.  This  can  be  easily  effected  by  cutting  two 
holes  in  the  window-shutter  of  a  dark  room  and  by  providing  them 


EXPERIMENTS  60,  61 


359 


with  adjustable  screens  of  coloured  and  colourless  glasses  respectively, 
as  in  the  annexed  diagram  (fig.  32).  Between  the  sources  of  light 
and  the  opal  surface  an  object,  e.g.  a  vertically  placed  ruler,  is 
interposed,  so  that  two  shadows  of  it  are  cast  upon  the  opal  surface, 
the  one  illuminated  by  the  coloured,  the  other  by  the  colourless 
light.  A  surprisingly  intense  contrast  colour  appears  in  the  really 
colourless  shadow,  the  intensity  of  the  contrast  varying  with  changes 


FIG.  32. 

in  the  relative  brightness  in  the  two  sources  of  light  with  which  the 
white  surface  is  illuminated. 

Exp.  60.  A  point  is  fixated  on  a  black  surface  between  two  red 
strips,  which  are  about  10  mm.  apart.  The  after-images  are  pro- 
jected on  to  another  black  surface.  The  observer  should  compare  the 
effects  of  projection  on  to  grey  and  white  surfaces,  and  the  effects  of 
laying  the  original  strips  on  a  grey  or  a  white  instead  of  on  a  black 
surface.  He  should  consider  what  theoretical  explanations  of  these 
effects  can  be  advanced. 

Exp.  61.  Pieces  of  the  same  grey  paper  are  to  be  placed  on 
colourless  backgrounds  of  different  brightness.  The  observer  notes 
the  production  of  brightness  contrast.  He  should  consider  how  the 
effects  of  changes  in  the  pupil  can  be  eliminated,  and  whether  the 
contrast  increases  or  diminishes  upon  fixation. 


360  EXPERIMENTAL  PSYCHOLOGY 

Bxp.  62.  A  circular  hole  about  1^  inches  in  diameter  is  cut 
in  a  large  white  card,  which  is  held  in  a  horizontal  position.  The 
observer  gazes  through  the  hole  on  to  a  sheet  of  colourless  or 
coloured  paper  placed  on  a  table  near  the  window.  He  observes 
the  alterations  in  brightness  of  the  underlying  paper  (e.g.  blue 
or  orange),  which  are  obtained  by  varying  the  inclination  of  the 
white  card. 

Bxp.  63.  A  point  is  fixated  on  the  margin  between  two 
adjoining  black  and  white  surfaces.  On  each  surface,  near  the 
common  margin,  a  short  strip  of  darkish  grey  paper  is  laid.  The 
observer  notes  the  effect  of  simultaneous  contrast  and  the  changes 
which  take  place  during  continued  fixation  of  the  point  (cf.  exp.  74). 
He  should  also  pay  attention  to  the  relative  brightness  of  the  two 
strips  in  the  after-image,  bearing  in  mind  the  theories  of  contrast. 


COLOUR  BLINDNESS. 

Exp.  64.  A  subject  may  be  most  easily  tested  for  colour  blind- 
ness by  means  of  Holmgren's  wools,  provided  that  the  following 
cautions  be  observed.  The  experimenter  must  never  mention  the 
name  of  a  wool ;  he  gives  the  test  wool  to  the  subject,  merely  asking 
him  to  select  wools  of  similar  colour.  The  experimenter  observes  not 
only  the  wools  which  the  subject  finally  selects,  but  also  those  which 
he  from  time  to  time  takes  up  and  rejects.  Both  highly  and  lowly 
saturated  wools  should  be  employed. 

Colour-blind  people  may  elude  detection  by  their  familiarity  with 
colour  nomenclature.  They  come  to  recognise  colours  to  which 
they  are  really  blind  by  differences  in  saturation  and  in  brightness. 
Cases  of  anomalous  colour  vision  without  absolute  blindness  may  be 
detected  by  the  above  method. 

The  following  matches  have  been  actually  made  for  red,  yellow, 
blue,  and  green  by  colour-blind  people.  Paper  discs,  corresponding 
to  the  right-hand  side  of  these  equations,  should  be  rotated  on  the 
colour- wheel  by  the  student — 

Scoteythrous    class    .        .        .  K  =    18°  Y  +  342°  Bk 
Photerythrous    „       .       .,.  ,  ,..  R  =  102°  Y  +  258°  Bk 

R,  =      5°W  +  355°  Bk 

B=    88°W  +  272°Bk 


EXPERIMENTS  65-67  361 

FLICKER. 

Bxp.  65.  The  observer  notes  the  phenomena  which  attend  the 
gradual  extinction  of  flicker,  as  a  white  sector  on  a  black  ground  is 
rotated  with  increasing  speed  upon  the  colour  wheel.  He  should  look 
for  Charpentier's  bands,  Fechner's  colours  (apparent  with  bright 
illumination),  the  coarse  flicker,  the  glitter,  the  fine  flicker,  and  the 
finally  complete  fusion  of  sensations. 

Bxp.  66.  A  card  of  black  and  white  sectors,  arranged  as  in 
fig.  33  to  illustrate  the  Talbot- Plateau  Law,  is  rotated  on  the  colour- 


Fra.  33. 

wheel.      The    various    rings    will    be    found   to  assume  the   same 
grey. 

ESTIMATIONS  OF  BRIGHTNESS. 

Exp.  67.  The  brightness  of  a  colour  is  to  be  measured  by  finding 
a  grey  background  on  which  it  becomes  invisible  when  seen  by  the 
peripheral  retina.  For  this  purpose  the  subject  fixes  his  eye  on  a 
spot  marked  upon  a  large  black  surface.  The  experimenter  introduces 
a  coloured  disc  (best  mounted  on  an  iron  rod),  moving  it  along  the 
black  surface  until  it  is  seen  by  the  subject  as  a  colourless  field. 
Then  various  shades  of  grey  are  presented  at  this  point  along  with 


362 


EXPERIMENTAL  PSYCHOLOGY 


the  colour  stimulus,  until  a  grey  is  formed  the  brightness  of  which 
appears  to  be  uniform  with  that  of  the  colour. 

This  determination  is  to  be  compared  with  the  estimation  of 
brightness  obtained  by  direct  comparison.  The  student  should  con- 
sider what  relative  effects  the  Purkinje  phenomenon  would  produce 
in  the  two  methods. 

Bxp.  68.  A  semicircle  of  grey,  and  one  of  a  colour  the  brightness 
of  which  is  to  be  tested,  are  arranged  so  that  they  form  a  circular 


FIG.  34.— An  Episcotister. 

The  width  of  the  two  open  sectors  C  C,  C  C,  can  be  varied  by  adjusting  the 
sliding  graduated  plates  H,  H.  These  plates  are  moved  by  the  arm  A, 
which,  is  movable  along  the  graduated  scale  D.  The  action  of  the 
arm  A  is  to  slide  the  rim  E,  and  the  outer  tube  of  which  it  forms  part, 
round  the  projecting  screw  F.  The  sectors  are  rotated  by  a  reliable 
motor,  a  belt  from  which  passes  over  B. 

vertical  field.  A  disc  composed  of  alternate  open  and  closed  sectors 
(fig.  34)  is  then  rotated  by  means  of  a  motor  before  this  field.  The 
observer  notes  whether  flicker  is  abolished  in  the  coloured  or  colour- 
less halves  of  the  field  at  the  same  moment.  He  replaces  the  grey 
by  other  shades  of  grey  until  flicker  disappears  in  both  halves  of  the 
circle  simultaneously.  Once  again  he  compares  his  results  with  the 
two  previous  methods. 


EXPERIMENTS  69-74 


363 


THE  INTRINSIC  LIGHT  OF  THE  EETINA.    DARK  ADAPTATION. 

Exp.  69.  The  gradual  effects  of  dark-adaptation  by  entering  a 
dark  room  should  be  carefully  noted.  In  absolute  darkness  only  the 
intrinsic  light  of  the  cerebro-retinal  system  remains.  The  nature  of 
this  light  and  the  details  occurring  therein  must  be  carefully  studied. 
When  the  darkness  is  less  complete  and  the  eye  has  become  dark 
adapted,  the  changes  in  relative  brightness  of  different  colours  are 
readily  noticeable. 

Exp.  70.  The  observer  determines,  in  bright  light,  the  value  of 
the  grey  which  results  from  rotating  fields  of  yellow  and  blue  on  the 
colour  wheel,  by  matching  it  with  a  grey  produced  by  the  rotation  of 
black  and  white  discs  on  another  colour  wheel.  Under  like  conditions, 
he  determines  the  grey  value  from  a  similar  fusion  of  red  and  green 
fields.  He  observes  what  alterations  occur  in  these  two  matches  when 
they  are  viewed  in  twilight  by  the  dark-adapted  eye. 

Exp.  71.  The  observer  notes  whether  differences  in  brightness 
exist  in  the  case  of  two  small  squares  of  the  same  coloured  paper,  one 
being  fixated  at  the  fovea,  the  other  stimu- 
lating the  peripheral  retina  ;  first  when  the 
eye  is  bright  adapted,  secondly  when  the  eye 
is  dark  adapted. 

COLOURED  WANING  OF  AFTER-!MAGE. 

Exp.  72.  The  positive  after-image  which 
follows  the  extinction  of  a  bright  light  is 
carefully  noted,   together  with   the  play  of 
FIG.  35.  colours    through    which   the   waning   image 

passes.     The   observer  should  note  in  what 

respects,  besides  in  hue  and  brightness,  it  differs  from  the  negative 
after-image. 

Exp.  73.  The  colours  obtained  by  spinning  Benham's  top 
(fig.  35)  are  observed.  The  spinning  disc  consists  of  black  lines  on  a 
white  ground.  The  colours  obtained  probably  have  an  origin  similar  to 
that  of  Fechner's  colours  and  the  coloured  waning  of  the  positive  after- 
image. 

SIMULTANEOUS  AND  SUCCESSIVE  INDUCTION. 

Exp.  74.  A  black  square  on  a  white  background  is  carefully 
fixated  by  the  aid  of  a  central  white  dot.  The  black  becomes  brighter, 


364  EXPERIMENTAL  PSYCHOLOGY 

the  white  darker  :  ultimately  both  merge  into  a  uniform  grey  (the 
"simultaneous  induction"  of  Hering).  The  observer  repeats  this 
experiment,  using  colourless  and  coloured  squares  on  colourless  back- 
grounds of  different  brightness.  He  notes  the  effects  in  the  after- 
image (the  "  successive  induction  "  of  Hering). 


EXEECISES  ON  CHAPTER  VIII 

G-ustatory  Sensations 

REACTIONS  OF  INDIVIDUAL  PAPILLAE. 

Exp.  75.  The  experimenter  makes  a  rough  outline  of  the 
subject's  tongue,  sketching  in  any  striking  landmarks  (ridges,  etc.) 
which  will  serve  to  locate  individual  papillae.  He  selects  six 
prominent,  easily  identifiable  papillae,  preferably  in  different 
situations,  and  notes  their  position  on  the  map  of  the  tongue. 

He  dries  the  subject's  tongue  with  a  piece  of  cloth  or  cotton  wool, 
and  investigates  the  reaction  of  these  papillae  to  distilled  water  and 
to  sweet,  bitter,  salt,  and  sour  solutions.  The  solutions  are  to  be 
applied  by  means  of  fine  brushes,  which  are  kept  in  water  and  are 
dried  before  being  dipped  in  the  solutions.  A  lens  should  be  used 
by  the  experimenter  in  order  to  insure  exact  application  to  the 
papillae.  The  experimenter  lightly  applies  the  brush  to  the  papilla 
for  two  seconds.  The  subject  does  not  withdraw  his  tongue  until  he 
has  an  answer  ready.  The  papillae  are  tested,  and  the  solutions  are 
employed,  in  irregular  order.  A  brief  rest,  preceded  by  rinsing  the 
mouth,  follows  each  application  of  the  brush. 

Record  is  made  of  (i.)  the  time  elapsing  between  application  of  the 
stimulus  and  development  of  the  sensation,  (ii.)  the  duration  of  the 
sensation,  (iii.)  the  nature  of  the  sensation,  (iv.)  any  changes  in  its 
character. 

Exp.  76.  The  experimenter  paints  a  papilla,  which  is  sensitive 
to  all  four  tastes,  with  a  10  per  cent,  solution  of  cocaine.  He  tests 
it  with  taste  solutions,  repeating  the  painting  and  testing  until  no 
further  effect  is  obtained. 

N.B. — Care  must  be  taken  not  to  apply  cocaine  to  a  wide  surface 
of  the  subject's  tongue,  as  some  individuals  are  peculiarly  susceptible 
to  the  dangerous  effects  of  too  much  cocaine. 

Exp.  77.  After  the  subject  has  chewed  some  gymnema  leaves,  or 
after  an  already  tested  papilla  has  been  painted  witli  a  solution  of 


EXPERIMENTS  78-80  365 

gymnemic  acid,   the  experimenter  again  tests  the  reaction  of  the 
papilla. 

MIXTURE,  COMPENSATION  AND  KIVALRY. 

Bxp.  78.  Two  different  taste  solutions  are  mixed  and  applied  to 
a  papilla  which  is  sensitive  to  the  two  tastes.  Both  weak  and  strong 
solutions  should  be  tested,  and  the  presence  of  compensation,  rivalry, 
or  of  an  altogether  new  sensation,  be  investigated. 

CONTRAST. 

Bxp.  79.  Having  dried  the  subject's  tongue,  the  experimenter 
applies  to  one  side  of  it  two  drops  of  a  taste  solution  of  the  nature  of 
which  the  subject  must  be  quite  ignorant.  The  solution  must  at 
first  be  so  weak  that  the  tasting  substance  is  incapable  of  stimulating 
the  end  organs.  It  is  applied  to  the  tongue  by  means  of  a  finely 
pointed  glass  tube.  The  solution  is  to  be  increased  in  strength  until 
with  successive  applications  the  subject  gives  correct  replies.  The 
subject  continues  to  hold  out  his  tongue  after  every  application, 
until  a  taste  sensation  is  developed  or  until  the  lapse  of  time  has 
assured  him  that  no  sensation  is  likely  to  arise.  In  every  case  the 
experimenter  notes  down  the  subject's  replies.  The  subject  rinses 
his  mouth  with  water  when  this  part  of  the  experiment  is  finished, 
and  rests  a  while. 

The  experimenter  now  attempts  to  induce  simultaneous  contrast 
by  applying  to  the  opposite  side  of  the  subject's  tongue  two  drops  of 
a  fairly  strong  solution  of  another  taste  (the  inducing  taste),  while 
on  the  other  side  he  applies  drops  of  the  taste  solution  previously 
used,  starting,  however,  with  pure  distilled  water  and  gradually 
increasing  the  strength  of  the  tasting  substance  until  its  taste  is 
recognised.  The  solutions  should  be  dropped  on  the  two  sides  of 
the  tongue  as  nearly  simultaneously  as  possible.  Care  must  be  taken 
that  they  do  not  mingle  on  the  tongue. 

The  effects  of  successive  contrast  may  be  demonstrated  by  applying 
the  inducing  taste  solution  to  the  tip  of  the  tongue.  After  two  or 
three  seconds  the  tongue  is  withdrawn  and  the  mouth  is  well  rinsed. 
The  experimenter  then  applies  distilled  water  or  various  strengths  of 
a  weak  previously  imperceptible  taste  solution  to  the  same  area. 

Olfactory  Sensations. 

CLASSES  OF  SMELLS. 

Bxp.  80.  The  subject  should  familiarise  himself  with  the  smells 
of  odorous  substances  which  are  at  his  disposal.  He  should  note 


366  EXPERIMENTAL  PSYCHOLOGY 

whether  tactile,  painful,  thermal,  or  gustatory  concomitants  of  the 
olfactory  sensation  are  present  in  each  case  ;  and  how  far  he  agrees 
with  Zwaardemaker's  classification. 

RESPIRATION  AND  SMELL. 

Bxp.  81.  The  subject  takes  several  rapid  deep  inspirations  and 
expirations,  so  that  he  is  subsequently  able  to  hold  his  breath  for 
about  twenty  seconds.  As  soon  as  he  starts  holding  his  breath  he 
closes  his  eyes,  and  brings  before  the  nose  a  bottle  of  strong  ammonia 
or  of  spirits  of  camphor.  He  observes  the  pricking  sensation  and  the 
entire  absence  of  olfactory  sensation.  When  the  breath  can  no  longer 
be  held,  he  closes  the  nostrils  with  the  fingers,  removes  the  bottle, 
and  takes  ever  so  gentle  an  inspiration,  observing  the  change  in 
sensation. 

FATIGUE. 

Exp.  82.  The  subject  familiarises  himself  with  the  character 
and  intensity  of  the  odours  of  chlorine  water,  animal  musk,  copaiba 
balsam,  heliotropin,  and  ether.  He  plugs  one  nostril  with  cotton 
wool  and  holds  under  the  other  a  bottle  containing  spirits  of  camphor, 
or  ammonium  sulphide,  or  tincture  of  iodine.  With  eyes  closed  he 
continues  smelling  the  bottle  until  the  odour  is  no  longer  perceived. 
Then  he  examines  the  four  odours  above  named  and  determines 
whether  or  not  they  have  changed  in  character  or  intensity.  He 
compares  the  different  results  according  as  the  nose  has  been  ex- 
posed to  spirits  of  camphor,  ammonium  sulphide,  or  tincture  of 
iodine. 

Exp.  83.  The  subject  should  observe  the  gradual  changes  in 
sensation,  as  nitrobenzol,  oil  of  camphor,  or  heliotropin  is  persistently 
smelled. 

MIXTURE,  COMPENSATION,  RIVALRY. 

Exp.  84.  By  using  a  double  form  of  Zwaardemaker's  Olfacto- 
meter  (fig.  8),  the  effects  of  simultaneously  presenting  two  odours 
to  the  nose  may  be  accurately  studied.  For  class  purposes,  however, 
it  may  suffice  for  the  experimenter  to  take  two  bottles  containing  the 
odours  and  to  pass  them  repeatedly  in  rapid  succession  before  the 
nostrils  while  the  subject  is  taking  a  slow  prolonged  inspiration. 
The  subject  must  be  already  quite  familiar  with  the  separate  odours, 
so  that  he  may  observe  whether  from  time  to  time  an  altogether  new 
sensation  occurs  when  they  are  presented  together. 


EXPERIMENT  85  367 

EXEECISES  ON  CHAPTEE  X 

Statistical  Methods 

THE  MEAN,  STANDARD  DEVIATION,  PROBABLE  ERROR,  ETC. 

Bxp.  85.  The  student  is  advised  to  work  out  their  values  for 

himself  from  the  series  of  measurements  given  in  the  first  of  the 
following  columns  : — 

v'                           d                     d*  v" 

190  -4        16  190 
197          +3         9  190 
196          +2         4  191 

191  -3         9  191 

195  +1         1  192 

192  -2         4  194 

194  0         0  195 

196  +2         4  195 
199          +5        25  196 

190  -4        16  1:96 

191  -3                      9  196 
196                         +2                      4  197 

195  +1                       1  199 


131     32  131  102 


m.v.     2-46         <T2  =  7-846 

o-  =2-8  Mdn  =  195. 

In  the  first  column  the  average  is  determined  ;  in  the  second,  the 
mean  variation  ;  and  in  the  third,  the  standard  deviation.  The  fourth 
column  gives  the  values  of  v'  in  numerical  order,  showing  the  median 
at  195  and  the  quartiles  at  191  and  196.  Half  the  difference  between 
the  quartiles,  namely,  2*5,  gives  the  semi-interquartile  range — a  third 
measure  of  the  variability  of  the  series. 

No  mean  is  of  any  value  unless  it  be  accompanied  by  a  figure 
expressing  the  variability  of  members  of  the  series.  But  only  one 
of  these  three  measures  of  variability  need  ever  be  calculated  for  a 
given  series.  The  most  usual  among  psychologists  is  the  mean  varia- 
tion. But  it  is  less  simple  than  the  semi-interquartile  range  and  less 
useful  than  the  standard  deviation,  from  which  the  probable  error,  E, 
of  the  mean  may  be  determined  by  the  formula — 

_  0-67450- 


368 


EXPERIMENTAL  PSYCHOLOGY 


N.B.  —  When  the  mean  variation  is  being  calculated,  it  is  convenient 
to  arrange  the  +,  zero,  and  —  values  of  d  separately,  thus  :  — 


+  3 

+  2 
+  1 
+  2 
+  5 
+  2 
+  JL 
+  16 


The  +  and  —  values  should 
be  numerically  equal,  if  the 
values  of  d  and  the  mean  have 
been  correctly  determined. 


-16 


Bxp.  86.  The  student  should  next  calculate  the  mean,  the 
standard  deviation,  and  the  probable  error  of  the  following  series : 
190,  198,  200,  192,  195,  200,  201,  195,  191,  194,  196,  199,  196.  He 
can  then  determine  the  relation  of  the  difference  between  this  and 
the  previous  mean  to  the  probable  error  of  the  difference  between  the 
means,  in  order  to  ascertain  whether  the  difference  is  with  any  high 
degree  of  probability  significant. 

CORRELATION. 

Bxp.  87.  The  correlation  between  the  following  thirteen  (=n) 
pairs  of  measurements,  vxt  vy,  is  here  determined,  the  means  of  the 
two  series  being  194,  145  and  their  standard  deviation  2m8,  3'4 
respectively. 

vx  vv  x  y  xy 

A  190  140  -4  -5  +20 

B  197  1'50  +3  +5  +15 

C  196  144  +2  -1  -  2 

D  191  140  -3  -5  +15 

E  195  144  +1  -1  -   1 

F  192  148  -2  +3  -   6 

G  194  146  0+1  0 

H  196  152  +2  +7  +14 

I  199  146  +5  +1  +5 

J  190  142  -4  -3  +12 

K  191  143  -3  -2  +6 

L  196  145  +2  0  0 

M  195  145  +1  0  0 

=     78 


Thus 


n  <rx 


=0-63, 


EXPERIMENT  87  369 

By  the  simpler  method  (page  130)  of  giving  orders  of  rank  to  A, 
B,  C,  etc.,  we  have 


Vr  Vy  d 

A  l|  11  00 

B  12  12  0  -0 

C  10  5|  4i  81 

D  3i       H  2"  16 

E  7J-  5i  2  16 

F  5  lT  6  144 

G  6        9|  3|  49 

H  10  13"  3  36 

I  13        9i  3^  49 

J  H        3"  ll  9 

K  3J       4  H  1 

L  10        7^  4  25 

M  7J       7|  0 


When,  as  here  happens,  two  or  more  individuals  tie  in  rank,  they 
are  each  given  a  figure  intermediate  between  the  ranks  which  they 
would  occupy  if  they  did  not  tie.  To  avoid  squaring  fractions,  2d 
has  been  squared  instead  of  d.  The  fraction  in  the  second  of  the 
alternative  formulae  quoted  on  page  130  must  therefore  be  divided  by 
four.  It  thus  runs  : — 

r=l- 
whence  r=0'70. 

The  discrepancy  between  the  results  of  the  two  methods  is 
unusually  great  in  this  particular  case,  and  is  partly  due  to  the  large 
number  of  tied  cases. 

N.B. — When  the  mean  and  standard  deviations  have  to  be 
calculated  for  long  series,  much  time  and  labour  may  be  saved  by 
taking  an  approximately  central  variate  (obtained  merely  by  casual 
inspection  of  the  series),  and  by  subtracting  each  member  of  the 
series  from  this  value.  Let  the  algebraical  sum  of  these  several 
differences,  divided  by  the  number  (n)  of  individual  values,  be 
represented  by  VL.  Then  vlt  when  added  (with  due  regard  of  sign) 
to  the  assumed  central  value,  will  be  found  to  give  the  average. 

So,  too,  let  the  sum  of  the  same  differences,  severally  squared,  be 
divided  by  the  number  of  individual  values,  and  let  this  sum  be 
represented  by  vy  Then  <r  =  '^vt,—vl2- 

Finally,  if  in  two  correlated  series,  2(x1i/1)  represent  the  sum  of 
24 


370 


EXPERIMENTAL  PSYCHOLOGY 


the  products  of  individual  pairs  of  differences  from  the  two  central 
values  (chosen  as  before),  and  if  vx,  vy,  a-X)  a-y  represent  the  values  of 
V,  and  o-  in  each  series,  then  the  formula  for  the  coefficient  of 
correlation  becomes 


By  such  means  a  considerable  saving  in  calculation  is  reached  in 
determining  the  mean,  the  standard  deviation,  or  the  coefficient  of 
correlation  for  lengthy  series. 


EXEKCISES  ON  CHAPTEK  XI 

Reaction  Times 

USE  AND  CONTROL  OF  THE  APPARATUS. 

Exp.  88.  Reaction  times  are  most  conveniently  and  accurately 
determined  by  interruptions  in  an  electric  circuit ;  a  current  being 
"  made  "  (or  "  broken  ")  at  the  moment  of  exhibition  of  the  stimulus, 
and  being  "broken"  (or  "made")  by  the  response  of  the  subject. 

The  response  usually  consists  in  lifting  the  finger  from  a  Morse 
telegraphic  key,  or  it  may  consist  in  lip  movement  or  in  vocalisation. 
For  the  two  last  modes  of  response  a  lip  key  or  a  voice  key  is  used, 


FIG.  36. 

in  which  the  electric  current  is  broken  by  movements  of  the  lip  or 
by  movements  of  a  membrane  thrown  into  vibration  by  the  voice. 

The  appliances  by  which  the  current  may  be  made  (or  broken) 
at  the  moment  of  exhibition  of  the  stimulus  are  of  various  kinds, 
depending  on  the  nature  of  the  stimulus.  The  simplest  available 
apparatus  is  a  Morse  key,  which,  when  sharply  and  suddenly  pressed 
upon,  serves  to  give  a  sound  stimulus  and  at  the  same  time  closes 
(or  breaks)  the  electric  circuit.  But  for  greater  convenience,  and  to 
insure  uniformity  of  stimulus,  the  sound  hammer  (fig.  36)  is 


EXPERIMENT  88 


371 


preferable.  In  this  instrument  a  steel  hammer,  H,  strikes  against  a 
steel  foot,  F,  being  drawn  to  the  latter  by  means  of  an  electro-magnet 
against  the  resistance  of  a  spiral  spring,  S. 

For  visual  reactions  various  forms  of  apparatus  exist,  involving 


FIG.  37. 


movement  of  a  shutter  or  a  pendulum,  and  so  designed  that  a 
current  is  made  or  broken  at  the  moment  of  exhibition  of  the 
stimulus.  It  is  essential  that  the  visual  stimulus  should  be 
presented  silently,  and  in  muscular  reactions  that  the  stimulus  be 
exposed  immediately  the  screen  begins  to  move.  Otherwise  the 

subject  is  apt  to  react  to  a 
noise  or  to  the  initial  move- 
ment of  the  screen,  instead 
of  to  the  desired  visual 
stimulus. 

FIG.  38.  Keaction  times  may  be  re- 

corded by  the  graphic  method  ; 

a  time  signal  (fig.  37)  and  a  time  marker  (fig.  38)  being  brought  to  bear, 
one  above  the  other,  upon  a  travelling  smoked  surface.  The  time 
signal  is  arranged  in  the  same  electric  circuit  with  the  two  pieces 
of  apparatus  above  described,  which  present  the  stimulus  and  receive 
the  reagent's  response,  respectively.  The  time  marker  records  the 
vibrations  of  an  electrically  driven  tuning-fork,  vibrating,  say,  50 
or  100  times  per  second.  By  this  means  a  tracing  like  the  following 
may  be  obtained  :  — 

-  __  ,  -  time  signal 


y\AAAAAAAAAAAAAAA/\f/me  marker 

FIG.  39. 

Hipp's  chronoscope,1  however,  is  a  far  more  convenient  instrument 
for  recording  reaction  times  (figs.  40,  41,  42).     The  clockwork  of 

1  This  instrument  is  undoubtedly  of  English  origin,  having  been  first  made  in 
1840  by  Wheatstone.  A  specimen  was  seen  by  Hipp  at  Karlsruhe  in  1842,  who 
subsequently  constructed  the  model  which  goes  by  his  name. 


372 


EXPERIMENTAL  PSYCHOLOGY 


the  chronoscope  is  started  by  pulling  on  one  of  the  two  hanging 
cords,  S'  S"  (fig.  40),  and  it  is  stopped  by  pulling  on  the  other  cord. 
It  is  driven  by  a  weight,  "W,  which  is  raised  by  a  key  fitting  into  the 
centre  of  the  lower  dial.  The  crown  wheels,  C'  and  C"  (fig.  41),  which 
lie  near  the  front  of  the  clock,  have  100  teeth.  The  balance  wheel,  E, 
lying  above  and  behind  the  crown  wheels,  is  regulated  by  a  small 
tongue  of  steel,  T,  which  is  thrown  into  movement  by  the  starting 
of  the  clock,  and  is  accurately  tuned  to  vibrate  1000  times  per  second. 

... ,%  This  steel  tongue  thus  allows  a  single 

tooth  of  the  balance  wheel  to  escape 
every  thousandth  part  of  a  second. 
The  two  dials,  D'  D",  are  divided 
into  hundredth  parts.  The  hand  of 
the  upper  dial  revolves  therefore 
ten  times  every  second,  and  each 
division  of  the  dial  corresponds  to 
a  thousandth  part  of  a  second,  i.e. 
to  1*.  The  hand  of  the  lower  dial 
is  geared  so  as  to  revolve  once 
every  10  seconds,  each  division  of 
this  dial  corresponding  to  one-tenth 
of  a  second,  i.e.  to  100^. 

While  the  clockwork  of  the 
chronoscope  continues  in  action, 
the  hands  of  the  dials  may  be 
arrested  or  started  at  any  moment 
by  means  of  the  electro -magnetic 
mechanism  at  the  rear  of  the  in- 
strument. For  this  purpose,  either 
the  upper  or  the  lower  pair  of  bob- 
bins, B'  W  (in  rare  cases  both  pairs) 
may  be  used  at  will ;  the  lower  or 
upper  spiral  springs,  S'  S"  (fig.  42), 
being  made  appropriately  tense  by 
adjustment  of  the  controlling  levers, 

H'  H",  which  are  placed  at  the  sides  of  the  instrument.  By  this 
arrangement,  after  the  current  has  ceased  to  flow  through  those 
bobbins  which  are  being  used,  the  armature,  A,  is  immediately 
released  from  contact  with  the  bobbins. 

The  movement  of  the  armature,  thus  brought  about  by  one  or 
other  pair  of  bobbins,  serves  to  vary  the  play  of  the  vertical  rod,  L, 
connected  with  the  armature,  A,  upon  the  upper  axis  or  spindle,  p  p, 
which  passes  horizontally  from  the  upper  dial  through  the  hollow 


FIG.  40. 


EXPERIMENT  88 


373 


axis  of  the  crown  wheels,  C'  and  C",  to  the  back  of  the  instrument. 
When  the  armature  is  pulled  down  (by  the  action  of  the  lower 
springs  or  magnets),  the  spindle,  p  p,  and  the  hand  of  the  upper  dial 
are  drawn  back,  so  that  a  small  cross-bar,  J  J,  fixed  on  the  spindle, 
fits  into  one  of  the  teeth  of  the  hinder  crown  wheel,  C",  through 
which  the  spindle  passes.  The  movements  of  this  crown  wheel  are 
thus  communicated  to  the  spindle  and  to  the  hands  of  the  dials. 


FIG.  41. 

When,  on  the  other  hand,  as  in  fig.  41,  the  armature  is  held  up  (by  the 
action  of  the  upper  springs  or  magnets),  the  cross  bar,  J  J,  of  the  upper 
spindle  no  longer  remains  in  the  teeth  of  the  crown  wheel,  C";  the 
spindle  and  the  hands  are  seen  to  come  forward,  so  that  the  cross-bar 
of  the  former  now  engages  in  the  other  crown  wheel,  C',  placed  slightly 
nearer  the  front  of  the  clock,  which  only  differs  from  the  former  in 
that  it  is  fixed  instead  of  being  movable. 


374 


EXPERIMENTAL  PSYCHOLOGY 


Thus,  in  the  former  position  of  the  spindle  the  hands  move, 
while  in  the  latter  they  are  thrown  out  of  action,  although  the  clock- 
work is  continuously  in  movement ;  and  as  these  two  positions  are 
determined  by  the  closure  or  interruption  of  an  electric  current,  we 
are  enabled,  by  putting  the  terminals,  F,  of  the  chronoscope  in  the  same 

circuit  with  the  rest  of 
the  reacting  apparatus,  to 
record  the  length  of  time 
in  which  the  dial  hands 
have  revolved  between  the 
exhibition  of  the  stimulus 
and  the  response  of  the 
reagent.  Thus  if  A  be 
(fig.  43)  the  sound  hammer, 
B  the  battery  or  other 
source  of  current,  C  the 
subject's  key,  and  D  be 
one  of  the  two  pairs  of 
terminals  of  the  chrono- 
scope ;  then,  when  the 
key  is  depressed,  the  cir- 
cuit will  be  closed  when 
the  stimulus  is  exhibited 
(i.e.  when  the  sound  ham- 
mer falls),  and  will  be 
broken  when  the  subject 

reacts.  With  this  arrangement,  those  two  terminals  of  the  chrono- 
scope must  be  chosen  which  allow  of  magnetisation  of  the  lower  pair 
of  bobbins.  If  now  the  clockwork  of  the  chronoscope  be  started, 
movement  of  the  clock  hands  is  effected  by  making,  and  is  arrested 


FIG.  42. 


FIG.  43. 


by  breaking,  the  circuit.  Supposing  that,  before  such  a  reaction 
experiment,  the  hands  of  the  lower  and  upper  dials  be  at  35  and  80 
respectively,  the  experimenter  records  their  position  as  3580.  If 
after  the  reaction  they  occupy  the  respective  positions  of  37  and 


EXPERIMENT  88 


375 


42,  he  subtracts  3580  from  3742,  and  obtains  the  reaction  time  162  in 
terms  of  o-. 

[This  is  the  simplest  arrangement  of  the  chronoscope ;  in  which 
the  act  of  exhibiting  the  stimulus  and  the  response  of  the  subject 
respectively  make  and  break  the  chronoscope  current.  It  is,  however, 
sometimes  desirable  that  the  act  of  exhibiting  the  stimulus  should 
break  the  chronoscope  current  instead  of  making  the  current,  in  which 
case  the  upper  instead  of  the  lower  bobbins  of  the  chronoscope  must 
be  employed  ;  so  that  when  the  current  is  broken  the  hands  of  the 
dials  are  instantly  put  into  action,  and  when  it  is  re-made  by  the 
subject  they  are  again  put  out  of  action.  In  this  case  the  subject 
reacts  by  closing  the  current.  But  for  accurate  work  it  is  undesirable 
that  the  response  of  the  subject  should  consist  in  making  a  contact 
(a  slightly  variable  error  being  necessarily  introduced  by  such 
procedure).  This  may  be  avoided  if  the  battery  current,  B  (fig.  44), 
be  given  the  choice  of  passing  through  (i.)  a 
circuit  of  raised  resistance  containing  the  upper 
bobbins  of  the  chronoscope,  D,  or  through  (ii.) 
an  alternate  circuit  of  lower  resistance  in  which 
are  placed  the  exhibiting  apparatus,  A,  and  the 
reacting  apparatus,  C.  The  resistance  in  (i.)-is 
raised  by  the  use  of  a  rheocord,  E.  In  this 
arrangement  the  current  flows  through  the 
higher  resistance  circuit,  i.e.  through  the 
chronoscope,  until  the  stimulus  is  exhibited  ; 
whereupon  the  lower  resistance  circuit  is  com- 
pleted and  the  current  in  the  chronoscope 
circuit  is  so  far  reduced  that  the  magnets  of 
the  chronoscope  no  longer  restrain  the  hands  from  moving.  As  soon 
as  the  subject  reacts,  the  lower  resistance  circuit  is  again  broken,  so 
that  the  current  must  needs  confine  itself  to  the  chronoscope  circuit 
and  thus  arrests  the  movement  of  the  hands.] 

The  chronoscope  requires  careful  treatment  in  order  to  ensure 
uniformity  and  accuracy  of  readings.  A  commutator  must  always  be 
introduced  into  the  chronoscope  circuit,  so  that  the  direction  of  the 
current  may  be  reversed  after  each  individual  reaction,  thereby 
preventing  permanent  magnetisation  of  the  soft  iron  core  within  the 
bobbins.  The  intensity  of  the  current  should  be  of  a  known  and 
constant  value.  For  this  purpose  a  rheocord  and  a  galvanometer  (or 
ammeter)  are  employed.  The  springs  of  the  chronoscope  must  be 
safeguarded  from  fatigue. 

The  reliability  of  the  chronoscope  must  be  tested  by  some  form  of 
control  instrument.  One  of  the  best-known  forms,  the  control 


FIG.  44. 


376 


EXPERIMENTAL  PSYCHOLOGY 


hammer  (fig.  45),  essentially  consists  of  a  hammer,  H,  which  during 
its  fall  successively  makes  and  breaks  (or  breaks  and  re-makes),  at 
I/,  L",  a  current  flowing  through  the  chronoscope.  The  actual  time 
occupied  in  the  fall  of  the  hammer  may  be  afterwards  determined  by 
attaching  a  piece  of  smoked  paper  to  the  hammer  and  by  recording 
on  it  the  vibrations  of  a  stationary  tuning-fork.  The  control  time 
may  be  altered  by  varying  the  position  of  the  weighted  counterpoise 
or  by  lowering  or  raising  the  electro-magnet,  B,  through  which  an 
independent  current  passes,  holding  up  the  hammer  until  the  time 
has  come  for  its  release. 

[So  long  as  alternate  circuits  (as  shown,  for  example,  in  fig.  44) 
are  not  employed  in  the  reaction  current,  the  error  of  the  chronoscope, 


FIG.  45. 

as  tested  by  the  control  instrument,  is  usually  the  same  for  longer  as 
for  shorter  intervals  of  time.  When  the  error  is  proportionate  to 
the  length  of  the  interval,  we  may  suspect  that  the  clockwork  runs 
faultily,  the  error  lying  perhaps  in  the  teeth  or  in  the  escapement. 

Assuming,  however,  that  the  clockwork  of  the  laboratory  instru- 
ment is  not  at  fault,  we  turn  our  attention  to  another  source  of 
error.  A  latent  period  of  time  necessarily  elapses  between  the  closure 
of  the  current  and  the  attraction  of  the  armature.  Another  latent 
period  elapses  between  the  breaking  of  the  current  and  the  release  of 
the  armature.  It  is  desirable  that  these  two  intervals  should  be 
equal.  They  are  each  due,  in  part,  to  mechanical  inertia,  to  which  is 
added  the  gradual  growth  of  magnetism  in  the  case  of  the  former, 


EXPERIMENT  88  377 

and  remanent  magnetism  in  the  case  of  the  latter  period.  That  is  to 
say,  magnetisation  does  not  at  once  reach  its  full  height,  nor  does  it 
abruptly  cease. 

These  two  intervals  may,  in  certain  cases,  be  of  considerable 
length,  relatively  to  the  time  of  running  of  the  clockwork.  They  are 
affected  by  changing  either  the  tension  of  the  springs  or  the  intensity 
of  the  current.  It  is  found  convenient  in  practice  to  vary  only  the 
current  intensity. 

Let  us  suppose  that  the  hands  are  running  when  the  chronoscope 
current  is  closed.  According  as  the  time  recorded  by  the  chronoscope 
exceeds  or  falls  short  of  the  value  of  the  true  control  time,  the  flow  of 
current  should  be  diminished  or  increased  by  increasing  or  decreasing 
the  resistance  of  a  rheocord  placed  in  the  chronoscope  circuit.  The 
mean  variation  of  the  error  may  be  often  reduced  by  regulating  the 
tension  of  the  springs  against  which  the  armature  of  the  chronoscope 
works.] 

The  chronoscope  should  be  controlled  before,  and  at  the  end  of, 
every  series,  and  in  the  middle  of  a  prolonged  series  of  reactions. 
The  mean  variation  of  a  series  of  control  tests  should  not  exceed 
one  hundredth  of  the  mean  of  the  recorded  times.  After  each 
individual  test  the  commutator  must  be  regularly  reversed,  so 
that  the  current  is  passed  in  either  direction  through  the  chrono- 
scope. 

Erroneous  readings  are  apt  from  time  to  time  to  arise  from 
improper  vibration  of  the  steel  tongue,  T,  which  controls  the  balance 
wheel,  E,  of  the  chronoscope.  Sometimes  it  vibrates  with  half  its 
proper  frequency,  sometimes  it  shows  other  forms  of  irregularity. 
After  a  little  experience,  however,  the  proper  pitch  of  the  tone,  which 
the  tongue  should  emit  before  a  control  time  or  reaction  time  is  taken, 
is  easily  recognised. 

Care  must  be  taken  to  interpret  the  position  of  the  more  slowly 
moving  index  of  the  chronoscope  dials  in  the  light  of  informa- 
tion yielded  by  the  other  index.  Supposing,  for  example,  that 
the  former  points  exactly  to  35  and  that  the  latter  points  to  98,  the 
reading  must  be  taken  as  3498,  not  as  3598.  Were  the  latter  the 
real  reading,  the  index  would  point  nearly  to  36,  instead  of  pointing 
to  35. 

The  experimenter  and  the  subject  are  now  in  a  position  to 
take  a  series  of  ten  control  times  with  the  control  hammer, — using 
the  upper  or  the  lower  electro-magnets  of  the  chronoscope,  which- 
ever are  required  for  the  ensuing  reaction  experiments.  When  the 
results  are  satisfactory,  they  should  proceed  with  the  following  ex- 
periment. 


378  EXPERIMENTAL  PSYCHOLOGY 

SIMPLE  REACTIONS. 

Exp.  89.  The  subject  should,  if  possible,  be  seated  in  a  quiet 
room,  remote  from  that  in  which  the  chronoscope  is  placed.  An 
assistant  should  be  with  him,  in  order  to  record  observations  of  the 
latter's  behaviour  during  reaction  (e.g.  premature  reactions)  and  to 
take  down  from  him  any  introspective  notes.  The  room  in  which 
the  subject  sits  contains  the  apparatus  exhibiting  the  stimulus,  the 
key  by  which  he  responds  to  it,  and  an  electric  signal  of  some  kind 
whereby  the  experimenter  is  able  to  warn  the  subject  to  prepare 
himself  for  the  reaction.  When  this  signal  sounds,  the  subject  gets 
ready,  and  if  a  finger  (or  Morse)  key  is  used,  he  places  his  finger 
lightly  on  it,  with  his  hand  and  arm  comfortably  supported. 

The  experimenter  gives  the  warning  signal,  when  his  apparatus  is 
ready  and  when  the  clockwork  of  the  chronoscope  has  been 
satisfactorily  started.  After  a  nearly  constant  interval,  preferably 
between  one  and  two  seconds,  the  stimulus  is  presented,  either  by 
electric  means  from  the  experimenter's  room,  or  by  the  assistant  who 
is  in  the  subject's  room.  If  the  assistant  presents  the  stimulus,  care 
must  be  taken  that  the  subject  cannot  see  or  hear  any  preparation  for 
movement  on  the  part  of  the  former.  In  auditory  reactions,  the 
source  of  sound  should  be  invisible  to  the  subject.  A  dozen 
preliminary  trials  should  always  precede  the  records  obtained  from  a 
hitherto  unpractised  subject. 

The  experimenter  takes  care  that  the  current  running  through 
the  chronoscope  remains  of  uniform  strength,  as  dictated  by  the 
previous  experiment  with  the  control  hammer.  He  is  careful  to 
avoid  any  permanent  magnetisation  of  the  electro-magnets,  by 
reversing  the  commutator  after  each  reaction,  and  by  only  allowing 
the  current  to  flow  through  the  chronoscope  when  absolutely 
necessary.  Before  the  first  and  every  subsequent  reaction,  he  takes 
down  the  times  registered  on  the  dials  of  the  instrument.  At  the 
close  of  the  series  of  reactions  he  subtracts  successive  values  from  one 
another,  finds  the  average,  the  mean  variation,  and  such  other 
constants  as  are  needful.  If  on  different  days  a  sufficient  number  of 
reactions  can  be  obtained,  a  curve  (or  rather  a  polygon)  may  be 
prepared  showing  the  distribution  of  individual  reactions. 

In  natural  reactions  the  subject  receives  no  instructions ;  in 
sensorial  reaction  he  is  told  to  think  only  of  the  expected  stimulus, 
and  not  to  attend  to  the  movement  until  he  has  received  the 
stimulus ;  in  muscular  reactions  he  is  told  to  concentrate  his 
attention  on  the  movement  with  which  he  is  about  to  respond,  and 
not  to  think  of  the  expected  stimulus. 


EXPERIMENTS  90,  91  379 

A  series  of  at  least  ten  (preferably  thirty  or  forty)  reactions  should 
be  obtained  for  each  mode  of  reaction.  They  must  be  preceded  by 
preliminary  practice  ;  and  they  should  be  followed  by  some  further 
reactions,  in  which  the  subject  attempts  to  record  his  mental  be- 
haviour by  introspection.  The  difficulties  of  introspection  may 
perhaps  be  lightened  if  the  subject  limits  himself  to  describing  in 
some  reactions  his  experiences  anterior  to  the  reception  of  the  stimulus, 
in  others  his  experiences  upon  its  reception,  and  in  others  his  experi- 
iences  during  and  after  the  motor  response  to  the  stimulus.  (Such 
"  fractionisation,"  however,  is  fraught  with  serious  dangers.)  He 
should  specially  attend  to  the  nature  of  his  imagery  and  to  the  part 
played  by  volition  in  the  various  forms  of  reaction. 

COMPOSITE  REACTIONS. 

Exp.  90.  Reactions  involving  recognition  are  merely  a  more 
complete  form  of  sensory  reactions. 

Reactions  involving  discrimination  may  be  most  easily  performed 
by  exhibiting  to  the  subject  one  or  other  of  a  number  of  known 
colours  and  instructing  him  not  to  react  until  he  has  clearly  discrimi- 
nated the  presented  stimulus  from  the  other  possible  stimuli. 

Reaction  times  involving  choice  may  be  performed  by  furnishing  the 
subject  with  two  reaction  keys,  one  for  a  finger  of  either  hand.  He 
is  told  that  either  a  red  or  a  blue  stimulus  will  be  exhibited,  and 
that  he  is  to  react,  say,  with  the  right  hand  to  red,  and  with  the  left 
hand  to  blue.  In  this  case  an  assistant  exhibits  the  stimuli,  always 
taking  care  that  they  follow  in  quite  irregular  order,  and  recording 
the  nature  of  the  stimulus  and  the  mode  and  details  of  the  subject's 
response.  The  subject  should  introspectively  determine  his  change 
in  attitude  with  increasing  practice. 

ASSOCIATIVE  REACTIONS. 

Exp.  91.  The  times  of  these  reactions  are  so  long  that  they 
may  be  roughly  studied,  in  default  of  specially  adapted  experiments, 
by  the  assistant  pressing  down  a  Morse  key  at  the  moment  when  he 
exhibits  or  utters  the  stimulus  word,  while  the  subject  lifts  his  own 
key  and  simultaneously  responds  with  the  associated  word.  Accurate 
results,  however,  require  an  apparatus,  enabling  a  word  to  be  exposed, 
and  causing  the  chronoscope  current  to  be  made  (or  broken)  at  the 
moment  of  exposure,  together  with  a  voice  key  or  lip  key  for  the 
subject,  whereby  the  current  is  broken  (or  remade).  A  series  of  free, 
or  partly  or  wholly  constrained,  association  reactions  may  be  taken  in 
irregular  order,  and  the  average  times  of  the  three  groups  calculated. 


380  EXPERIMENTAL  PSYCHOLOGY 

EXEECISES  ON  CHAPTEES  XII.  AND  XIII 

Memory 

THE  MEMORY  IMAGE. 

Exp.  92.  It  is  assumed  that  the  subject  has  already  familiarised 
himself  with  the  general  nature  of  memory  images,  by  attempting  to 
revive  recent  or  familiar  scenes  or  actions.  He  now  rests  his  eye  on  a 
uniform  black  screen.  After  a  minute  the  experimenter,  opening  a 
window  in  the  screen  (or  by  other  means),  suddenly  exposes  for  about 
ten  seconds  a  light  or  dark  grey  disc  upon  a  background  of  medium 
greyness.  When  this  exposure  is  ended,  and  the  window  has  been 
closed,  the  subject  continues  dreamily  to  rest  his  eyes  on  the  black 
screen,  and  he  notes  whether  there  is  a  single  or  repeated  spontaneous 
revival  (perseverance)  of  the  memory  after-image,  whether  the  image 
can  be  reproduced  volitionally,  or  whether  no  visual  image,  worthy 
of  the  name,  is  representable.  Some  little  practice  is  often  neces- 
sary for  the  subject  to  obtain  good  results.  His  success  will  be 
more  assured  if  the  screen  be  provided  with  blackened  side  wings, 
so  that  no  external  objects  distract  his  gaze.  Care  must  be  taken 
not  to  confuse  the  sensory  with  the  memory  after-image  (page 
148). 

The  experiment  may  be  modified  by  similarly  exposing  a  second 
grey  disc,  somewhat  lighter  or  darker  than  the  first,  the  two  exposures 
being  separated  by  an  interval  of  half  a  minute.  The  subject  en- 
deavours, when  the  second  exposure  is  over,  to  compare  the  two  greys, 
observing  to  what  extent  he  makes  use  of  the  memory  image  of 
each. 

Exp.  93.  In  a  quiet,  preferably  darkened,  room  the  subject  listens 
for  about  twenty  seconds  to  a  tone  uniformly  and  continuously  sound- 
ing from  some  form  of  whistle.  After  the  tone  has  ceased,  he  observes 
the  memory  image. 

The  subject  should  notice  in  these  experiments  that  the  memory 
image  only  appears  and  can  only  be  held  fast  for  a  brief  time.  Its 
characters  should  be  compared  with  those  of  the  original  presenta- 
tion. He  should  observe  the  feeling  of  tension  in  the  head  and 
any  changes  in  the  localisation  of  this  feeling,  during  the  exposure 
of  the  disc  and  during  the  appearance  of  the  memory  image.  The 
effects  of  momentary  and  prolonged  exposures  should  be  com- 
pared. 


EXPERIMENTS  94,  95  381 

THE  CLASSIFICATION  OF  ASSOCIATIONS. 

Exp.  94.  The  experimenter  prepares  a  list  of  thirty  words, 
choosing  them  so  that  all  kinds  of  imagery  (visual,  motor,  etc.),  are 
represented.  By  the  aid  of  convenient  apparatus,  they  are  success- 
ively exhibited  to  the  subject,  who  takes  care  each  time  to  express  the 
first  idea  that  occurs  to  him.  The  association  times  and  the  words 
returned  are  noted  by  an  assistant. 

The  experimenter  classifies  the  associations  according  to  the  scheme 
on  page  152,  obtaining  all  possible  help  from  introspective  analysis  by 
inquiry  after  every  answer.  He  then  observes  whether  any  light  is 
thrown,  by  introspection  and  by  the  speed  and  nature  of  the  replies, 
on  a  predominant  type  of  imagery,  or  on  other  individual  peculiarities. 

If  time  permits,  the  experiments  described  on  pages  145-146,  and 
the  serial  method  mentioned  on  page  151,  may  be  performed.  The 
special  point  of  interest  in  ,these  experiments  is  the  great  individual 
differences  they  disclose. 

METHODS  OF  MEASURING  MEMORY.1 

THE  SAVING  METHOD. 

Exp.  95.  Each  member  of  the  class  prepares  a  series  of  twelve 
senseless  three-letter  syllables,  taking  care — 

(a)  that  two  consecutive  syllables  do  not  form  a  sensible  word  ; 

(6)  that  the  same  vowel  is  not  repeated  in  two  consecutive 
syllables  ; 

(c)  that  no  two  letters  of  one  syllable  recur  in  another  syllable  of 
the  same  series  ; 

(d)  that  the  final  letter  of  one  syllable  is  not  the  initial  letter  of 
the  next. 

These  syllables  are  to  be  written  out  in  large  printed  characters,  one 
beneath  the  other,  each  series  on  a  separate  sheet  of  paper,  which  is 
then  handed  by  the  writer  to  his  neighbour.  At  a  given  signal  each 
person  begins  silently  to  learn  the  series  by  the  learning  method.  A 
noiselessly  swinging  pendulum  (or  a  metronome)  ensures  a  constant 
rate  of  reading.  At  the  first  correct  reproduction  the  experiment  is 
stopped.  The  repetitions  are  counted  at  the  close  of  the  experiment 
by  observing  the  number  of  discs  of  cardboard,  one  of  which  is  dropped 
from  the  hand  of  the  subject  after  each  repetition.  After  a  given 

1  An  elementary  class  cannot  be  expected  to  give  sufficient  time  to  experiment 
and  to  preliminary  practice,  in  order  to  obtain  quantitative  results  of  any  value. 
They  should,  however,  familiarise  themselves  with  the  experimental  methods. 


382  EXPERIMENTAL  PSYCHOLOGY 

interval,  say  one  hour,  the  series  is  relearnt  as  before,  and  the  saving 
in  repetitions  is  noted. 

THE  SCORING  METHOD. 

Bxp.  96.  The  following  is  Miiller's  arrangement  of  apparatus  for 
this  method.  The  student  should  familiarise  himself  with  the  use  of 
such  instruments  as  the  laboratory  possesses  which  are  available  for 
the  purpose,  should  the  laboratory  not  possess  the  apparatus  which  is 
here  described. 

In  this  method  the  syllables  are  written  on  a  cylinder,  and  during 
the  rotation  of  the  cylinder  on  its  horizontal  axis  they  are  success- 
ively exposed  to  the  subject  before  a  small  window  in  a  screen. 
During  reading,  the  subject  gives  so  far  as  possible  equal  attention 
to  each  syllable,  and  accents  them  as  directed.  In  the  interval 
between  the  last  reading  and  the  re-exhibition,  he  is  careful  to  avoid 
thinking  over  his  lesson. 

1  The  re-exhibition  apparatus  consists  of  a  polyhedral  prism,  rotating 
on  a  horizontal  axis,  on  each  side  of  which  is  printed  the  first  syllable 
of  some  pair  belonging  to  the  series  of  syllables  already  learnt.  This 
prism  is  concealed  from  the  subject's  eye  by  a  releasable  screen,  which 
falls  when  the  subject  breaks  an  electric  circuit,  thereby  exposing  a 
single  syllable  printed  on  the  presented  side  of  the  prism.  By  closing 
another  electric  circuit,  the  falling  screen  sets  a  Hipp's  chronoscope 
in  action  at  the  moment  of  exposure  of  the  syllable.  The  subject,  in 
giving  out  his  reply,  stops  the  movement  of  the  chronoscope  owing  to 
its  electrical  connection  with  a  lip  key  or  a  voice  key  which  moves 
when  he  speaks.  The  chronoscope  readings,  thus  obtained  for  the 
various  pairs  of  syllables,  are  indeed  not  identical  with,  but  they  may 
be  taken  as  a  measure  of,  the  time  occupied  in  reviving  the  associated 
syllable. 

ECONOMICAL  METHODS  OF  LEARNING. 

Exp.  97.  The  experimenter  writes  in  a  clear  hand  a  dozen  verses 
of  poetry  on  the  blackboard.  The  subjects  note  the  number  of  silent 
repetitions  necessary  to  learn  them  by  the  entire  method,  counting  the 
repetitions,  as  in  Exp.  95,  by  dropping,  as  before,  a  small  disc  of  card- 
board from  the  hand  at  each  repetition.  After  five  minutes'  rest,  the 
number  of  necessary  repetitions  is  determined  in  order  to  learn  a 
further  series  of  equally  difficult  verses  by  the  sectional  method,  the 
series  being  learnt  in  two  equal  sections.  After  similar  intervals, 
a  third  series  is  learnt  by  the  entire  method,  a  fourth  series  by  the 
sectional  method,  in  which  the  series  is  divided  into  three  (four,  or 
six)  sections,  and  a  fifth  series  by  the  entire  method. 


EXPERIMENT  98  383 

The  class  should  compare  the  economy  in  repetitions  for  these 
three  methods,  and  make  careful  introspective  records  of  their 
experiences.  They  should  consider  what  further  factors  need  to  be 
investigated,  in  order  to  determine  more  accurately  the  relative 
economy  of  methods  of  learning. 


EXEKCISES  ON  CHAPTER  XIV 

Muscular  Work 

MUSCULAR  FATIGUE. 

Bxp.  98.  The  dynamometer  and  the  ergograph  are  the  two 
instruments  which  have  been  used  in  the  psychological  study  of 
muscular  fatigue.  Observations  have  been  confined  to  local,  they 
have  not  extended  to  general,  bodily  fatigue. 

The  dynamometer  registers  the  squeeze  or  pull  of  the  hand  or 
finger  against  a  steel  spring.  It  may  be  used  in  two  ways  :  either  to 
record  the  maximal  force  of  a  momentary  muscular  contraction  under 
varying  conditions,  or  to  record  the  variations  in  that  force  when 
prolonged  effort  is  made  to  maintain  a  state  of  maximal  contraction. 
The  instrument,  however,  has  various  drawbacks.  In  the  first  place, 
the  maximal  force  varies  with  the  suddenness  with  which  the  con- 
traction is  made.  Secondly,  the  pressure  of  the  bar  or  handle  against 
the  skin  is  apt  to  be  very  painful,  and  therefore  to  inhibit  the  full 
force  of  the  contraction.  Thirdly,  the  movement  required  is  so 
complex  that  there  is  no  security  against  bringing  into  use  different 
muscles,  or  against  contracting  different  muscles  to  varying  extents, 
at  different  times  in  the  course  of  the  investigation.  A  modified 
form  of  the  dynamometer  is  employed  and  figured  in  exp.  155. 

The  ergograph  (fig.  46)  is  especially  adapted  for  the  study  of 
simple  movements  in  which  very  few  muscles  are  involved.  The 
most  usually  studied  movement  consists  in  extending  and  flexing  the 
middle  finger.  It  is  the  essence  of  a  good  instrument  that,  by  the 
rigid  fixation  of  the  arm,  hand,  and  other  fingers,  all  auxiliary  move- 
ments be,  so  far  as  possible,  excluded,  and  that  a  minimum  of 
discomfort  attend  the  recording  of  the  ergogram.  In  most  instruments 
the  work  is  done  (that  is  to  say,  the  weight  is  lifted)  with  the  hand 
placed  palm  upwards  ;  in  others  (cf.  fig.  46)  the  hand  rests  with  the 
palm  downwards. 

In  Krapelin's  ergograph  (fig.  46)  the  arm  is  placed  on  a  firmly 
fixed  platform  F,  and  is  clamped  by  the  cross-bars  A  and  B.  The 


EXPERIMENTAL  PSYCHOLOGY 


middle  finger  lies  midway  between  the  first  and  third  fingers,  which 
are  separated  from  it  by  the  right-angled  plates   C  and  D.     It  is 


FIG.  46. — Krapelin's  Ergograph. 

comfortably  but  firmly  secured  by  means  of  screws  within  the  box  E, 
and  is  free  to  execute  up-  and  downward  movements.  The  several 
parts  which  thus  fix  the  fingers  are  movable  and  are  graduated,  so 


EXPERIMENTS  99,  100  385 

that  the  hand  can  be  repeatedly  replaced  in  the  same  position.  The 
movements  of  the  middle  finger  are  communicated  by  the  steel  ribbon 
H  to  the  axis  of  J,  and  thence  by  the  cord  T  to  the  lever  L,  which  is 
raised  against  the  action  of  the  spring  S.  L  records  the  finger  move- 
ments on  a  travelling  smoked  surface.  The  upright  board  X  is 
10  feet  or  more  in  height.  At  its  top  it  bears  a  pulley,  round  which 
passes  a  long  wire,  attached  at  one  end  N  to  the  spirally  grooved 
surface  J,  and  at  the  other  end  to  the  (variable)  weight  W  placed  on 
the  other  side  of  the  board. 

Flexion  of  the  middle  finger  rotates  J,  and  thus  raises  W,  but  the 
axis  of  J  is  so  arranged  that  no  reverse  movement  of  J  or  lowering 
of  W  occurs  during  relaxation  of  the  finger.  In  virtue  of  this  con- 
trivance, it  is  easy  to  calculate  the  total  height  to  which  W  has  been 
raised  during  any  experiment,  by  means  of  a  vertical  scale  placed  on 
the  rear  surface  of  X. 

In  a  series  of  preliminary  experiments,  the  weight  which  has  to  be 
raised  must  be  regulated  according  to  the  physique  of  the  subject  and 
according  to  the  prescribed  frequency  of  movement. 

To  obtain  an  ergogram  of  the  usual  form,  as  shown  in  fig.  4,  the 
weight  must  be  relatively  heavy.  A  metronome  is  used  to  mark  the 
rate  of  rhythm.  The  subject's  eyes  are  screened  from  the^  smoked 
surface,  on  which  the  height  and  number  of  his  contractions  are 
recorded.  When  the  height  and  number  of  the  contractions,  or  the 
total  height  through  which  the  weight  has  been  lifted  in  a  known 
time,  are  known,  the  amount  of  work  can  readily  be  expressed  in 
units  of  work  (kilogram- metres). 

MUSCULAR  PRACTICE. 

Bxp.  99.  It  is  not  difficult  to  devise  experiments  which  shall 
test  the  degree  of  practice  in  speed  and  accuracy  of  movement. 
Learning  how  to  typewrite,  or  aiming  successive  balls  at  the  centre  of 
a  target,  are  examples.  The  target  may  be  covered  with  two  sheets  of 
paper,  between  which  is  inserted  a  piece  of  carbon  paper.  By  such 
means  the  effects  of  increasing  practice,  fatigue,  and  the  extent  of 
retention  of  practice  can  be  quantitatively  determined. 

Mental  Work. 

METHODS  OP  TESTING. 

Exp.  100.  A  method  of  determining  the  spatial  threshold  is 
described  in  exp.  103. 

The  chief  difficulty  of  the  "  combination,"  "  letter-erasing "  and 

25 


386  EXPERIMENTAL  PSYCHOLOGY 

"  learning  "  tests  lies  in  evaluating  the  results.  The  errors  may  be 
either  of  commission  or  of  omission,  and  it  is  not  easy  to  apportion 
the  "  bad  marks  "  which  each  kind  of  error  should  deserve.  Some  in- 
vestigators allow  only  half  a  bad  mark  to  an  error  of  transposition  in 
the  learning  test ;  others  graduate  the  mark  according  to  the  extent 
of  transposition. 

In  the  combination  test,  it  is  essential  that  the  material  should 
be  of  constant  interest  and  difficulty  for  different  subjects,  and  for  the 
same  subject  at  different  times.  An  approach  to  uniformity  may  be 
attained  by  confining  the  omitted  words  to  some  definite  part  of 
speech,  e.g.  to  verbs. 

In  the  letter-erasing  tests,  the  material  should  again  be  of  constant 
interest  and  familiarity.  This  ideal  is  best  reached  in  the  case  of 
adults  by  using  nonsense  words,  or  in  the  case  of  children  by  using 
a  foreign  language  which  is  known  to  be  absolutely  meaningless  to 
them.  If  pages  of  nonsense  words  are  specially  prepared,  it  is  well 
to  arrange  the  words  so  that  every  half  page  contains  a  constant 
number  of  examples  of  the  letter  which  the  subject  is  enjoined  to 
erase. 

The  methods  employed  in  the  learning  test  have  been  already 
discussed  on  pages  153-156. 

For  the  simple  addition  of  the  calculation  tests,  special  books  of 
figures  (Rechenhefte)  have  been  prepared  under  Kriipelin's  direction. 
The  subject  adds  each  figure  to  the  next  and  writes  down  the  result. 
He  then  starts  afresh  and  adds  the  next  pair  of  figures,  and  so  on. 
He  makes  a  mark  at  the  figure  reached  whenever  the  time  signal  is 
sounded. 

These  books  are  not  so  suitable  for  multiplication,  as  special 
arrangements  are  necessary  so  that  the  various  pairs  of  figures 
multiplied  shall  be  of  fairly  uniform  difficulty. 

Satisfactory  experimental  results  can  hardly  be  expected  from 
students  whose  opportunities  for  investigation  are  confined  to  the 
hours  of  class  laboratory  work.  In  a  few  initial  experiments  it  is 
impossible  to  eliminate  the  enormous  influence  of  accommodation  and 
practice.  Nevertheless,  the  student  cannot  be  too  strongly  urged  to 
familiarise  himself  with  some,  at  least,  of  the  methods  described  in 
the  text,  and  to  preserve  a  record  of  his  introspections  made  after 
submitting  himself  to  the  tests. 


EXPERIMENT  101  387 

EXEECISES  ON  CHAPTEK  XV 

Psycho-physical  Methods 

METHOD  OF  MEAN  ERROR. 

Bxp.  101.  The  experimenter  should  apply  this  method  to  an 
investigation  of  the  conditions  affecting  the  accuracy  with  which 
the  subject  can  make  one  line  equal  to  another.  An  inexpensive  form 
of  apparatus  for  class  work  is  one  in  which  two  white  threads  are 
displayed  upon  a  black  screen,  the  length  of  either  being  adjustable 
by  an  arrangement  which  the  subject  can  easily  manipulate  behind 
the  screen.  A  series  of  ten  observations  should  be  taken  under  each 
of  the  following  conditions  :  the  standard  lying  (a)  to  the  right  of, 
(6)  to  the  left  of,  the  variable  ;  and  a  further  series  should  be  taken 
to  show  any  differences  dependent  on  whether  the  subject  has  to 
shorten  or  lengthen  the  variable  line  in  order  to  make  it  equal  to 
the  standard.  The  apparatus  should  be  held  so  that  the  plane  of 
the  screen  cuts  the  line  of  vision  at  right  angles ;  that  is  to  say,  it 
should  be  held  vertically  in  front  of,  or  horizontally  immediately 
below,  the  eyes. 

A  few  preliminary  practice  experiments  should  precede  the  actual 
record  of  results.  Every  group  of  ten  observations  should  be 
divided  into  two  sub-groups,  each  of  five  observations,  and  the  order 
of  taking  the  various  sub-groups  should  be  so  arranged  that  the 
influences  of  practice  and  fatigue  may  be  nearly  constant  for  each 
group.  Within  the  limited  time  available  in  class  work,  the  com- 
plete equalisation  of  conditions  is,  of  course,  impossible. 

The  experimenter  determines  the  constant  and  average  error  and 
the  mean  variable  error  (a)  from  the  whole  of  the  results  obtained, 
and  also  from  the  results  obtained  when  the  variable  lies  (6)  to  the 
left  and  (c)  to  the  right  of  the  standard  line.  He  may  thence  deduce 
the  space  error.  He  should  also  determine  the  constant  error  accord- 
ing as  the  variable  has  been  (d)  shortened  or  (e)  lengthened  by  the 
subject. 

Examples  of  the  application  of  the  Limiting  Method  are  given 
in  exp.  102,  of  the  Constant  Method  in  exp.  121,  and  of  the  Serial 
Method  in  exp.  103,  and  elsewhere. 


388  EXPERIMENTAL  PSYCHOLOGY 


EXEECISES  ON  CHAPTEE  XVI 

Weight 

THE  SIZE-WEIGHT  ILLUSION. 

Bxp.  102.  The  experimenter  employs  the  series  of  canisters  (fig. 
47)  provided  in  the  laboratory,  one  of  them  being  half  the  breadth 
or  half  the  height  of  the  others.  This, 
loaded  with  shot  so  as  to  weigh,  say,  200 
grams,  serves  as  the  standard,  while  the  larger 
canisters,  differently  weighted,  serve  as  the 
variables.  A  metronome  or  silently  swing- 
ing pendulum  may  be  used  to  mark  the  rate 
with  which  the  subject  must  raise  and  lower 
each  canister,  and  to  preserve  a  constant 
interval  between  handling  each  member  of 
the  various  pairs  of  canisters.  A  horizontal 

FIG.  47.-The  small  can-  T*  ^°ul(?  be  sfcf<*ed>  "ay,  six  inctes' 
ister  is  employed  with  above  the  height  °f  the  camster  80  as  to 
the  larger  ones  in  measur-  ensure  a  constant  height  of  lift.  The  series 
ing  the  "size-weight"  of  canisters  is  screened  by  the  experimenter 
illusion.  Only  two  of  from  the  subject's  view.  The  experimenter 
the  larger  canisters  are  successively  places  the  canisters  so  that  the 
here  figured ;  in  actual  subject  iifts  each  canister  from  the  same 
2T£^  n^r  fa  spot,  sometimes  the  constant,  sometimes  the 
necessary.  The  larger  variable,  is  lifted  first.  In  this  way  the 
canisters  are  to  external  space  and  time  errors  (pages  203,  266)  are 
appearance  exactly  simi-  avoidable.  The  subject  grasps  each  canister 
lar,  but  they  are  differ-  by  the  body,  not  by  the  handle, 
ently  weighted  with  shot,  The  experimenter  first  plans  a  brief 
:r^  -ies  of  pairs  of  lifts  by  the  limiting 
lifted  by  one  finger,  method  in  order  roughly  to  determine 
which  is  inserted  be-  the  variable  weight  which  appears  to  the 
neath  the  handle.  subject  equal  to  the  standard.  In  these 

preliminary  experiments  the  variables  should 

range  between  280  and  400  grams,  and  differ  by  increments  of  20 

grams. 

Then  a  series  of  experiments  is  conducted  either  by  the  method 

of  serial  groups  or  by  the  constant  method.     The  beginner  should 

choose  the  former,  which  has  already  been  sufficiently  described  on 

page  209,  and  is  further  exemplified  in  exp.  103. 


EXPERIMENT  103  389 

[The  more  advanced  student  may  adopt  the  constant  method. 
Here  the  variable  canisters  differ  by  increments  of  10  grams,  and  the 
value  of  the  central  variable  is  that  which  (as  determined  in  the 
preliminary  experiment)  appears  approximately  equal  to  the  smaller 
canister,  i.e.  the  constant.  Five  variables  should  be  used,  and  each 
should  be  presented  in  irregular  order,  say  eight  times,  with  the 
constant. 

The  subject's  answers,  "heavier,"  "equal,"  "doubtful,"  or 
"  lighter,"  should  uniformly  refer  to  the  second  canister  lifted.  The 
experimenter  divides  the  "equal"  and  "doubtful"  answers  equally 
between  the  "heavier"  and  "lighter"  answers  (page  214).  He  then 
determines  (by  one  of  the  methods  alluded  to  on  page  215)  the  value 
of  the  canister  which  may  be  expected  to  give  equal  numbers  of 
"heavier"  and  "lighter"  answers.  This  is  evidently  the  weight  of 
the  variable  which  appears  to  be  equal  to  that  of  the  constant.  We 
have  thus  measured  the  size-weight  illusion.] 


EXEECISES  ON  CHAPTER  XVII 

Local  Signature 

THE  SPATIAL  THRESHOLD. 

Bxp.  103.  The  subject  rests  his  arm  comfortably  on  the  table, 
extensor  surface  downwards.  The  experimenter  marks  in  ink  a 
point  on  the  subject's  forearm,  which  is  to  indicate  the  middle  of  the 
region  to  be  investigated.  Having  found  a  distance  which  is  just 
sufficient  to  be  decidedly  above  the  subject's  spatial  threshold,  the 
experimenter  applies  the  compass  points,  thus  separated,  to  the 
subject's  arm  for  about  two  seconds.  The  latter,  having  his  eyes 
closed,  decides  whether  he  is  being  touched  by  two  points  or  by  one. 
The  compass  should  be  applied  ten  times  with  both  points  touching, 
and  ten  times  with  a  single  point  touching  the  skin,  in  irregular  order. 
Then  the  distance  between  the  points  is  to  be  reduced  by  five 
millimetres,  and  another  series  of  twenty  stimulations  is  begun.  The 
distance  is  in  this  way  reduced,  until  two  wrong  answers  in  ten  are 
obtained  for  the  answers  to  double  touches.  This  may  be  conven- 
tionally regarded  as  the  threshold. 

Care  must  be  taken  to  apply  the  two  points  simultaneously,  and 
with  equal  and  constant  pressure.  When  one  point  is  used,  it  should 
be  applied  near  one  or  other  of  the  spots  to  which  the  two  points  are 
applied. 


390  EXPERIMENTAL  PSYCHOLOGY 

The  subject  should  carefully  note  his  experiences  and  take  care 
that  they  are  recorded.  Sensations  of  cold  should  be  avoided.  The 
subject  should,  in  particular,  analyse  his  experiences  near  the 
threshold.  A  second  series  of  experiments  should  be  made,  so  that 
he  may  discover,  so  far  as  possible,  the  basis  of  his  improvement 
with  practice.  A  third  series  may  be  made,  in  which  the  subject  is 
told  each  time  by  the  experimenter  whether  his  answer  is  correct  or 
not.  Here  he  should  try  to  find  the  reasons  for  his  wrong  answers, 
and  the  experimenter  should  note  the  results  of  this  modification  of 
the  experiment. 

It  is  worth  while  to  bear  in  mind  the  proposal  which  has  been 
put  forward  by  Binet,  that  subjects  are  divisible  into  three  classes, 
according  to  their  behaviour  in  this  experiment.  The  simplistes  only 
record  a  double  touch  as  such,  when  two  distinct  tactile  sensations 
are  present.  They  show  an  abrupt  transition  from  answers  which 
are  all  correct,  to  answers  many  of  which  are  wrong.  They  never 
err  in  describing  a  single  touch  as  double.  The  interpretateurs  make 
ample  use  of  inference,  and  avail  themselves  of  the  difference  between 
the  effects  produced  by  double  stimulation  and  those  produced  by 
single  stimulation,  even  when  a  double  touch  is  not  actually  experi- 
enced (page  232).  Their  threshold  is  less  definite.  The  distraits  are 
those  who,  owing  to  their  liability  to  distraction,  are  apt  to  confuse 
the  difference  between  single  and  double  touches,  and  thus  sometimes 
describe  single  as  double  touches.  This  latter  illusion,  however,  is 
certainly  not  always  due  merely  to  lack  of  attention  ;  its  causation 
requires  future  investigation. 

Exp.  104.  The  experimenter  takes  the  compass,  the  points  of 
which  are  set  at  a  distance  2  cm.  apart,  and  draws  it  with  uniform 
movement  and  pressure  across  the  cheek  of  the  subject  from  ear  to 
lip.  The  subject  observes  the  changes  in  apparent  distance  between 
the  points  and  in  their  apparent  rate  of  movement. 

ARISTOTLE'S  EXPERIMENT. 

Exp.  105.  The  subject  places  his  hand  palm  upwards  on  the 
table,  and  the  experimenter  crosses  the  subject's  ring  finger  over  the 
middle  finger.  The  experiment  consists  in  simultaneously  touching 
the  adjacent  sides  of  the  tips  of  the  crossed  fingers  with  a  single 
object.  This  is  done  by  the  experimenter,  the  subject  being  blindfold 
and  ignorant  whether  he  will  be  touched  by  one  or  by  two  objects. 

Several  modifications  of  the  experiment  have  been  described. 
Instead  of  touching  the  ulnar  side  of  the  middle  and  the  radial  side 


EXPERIMENT  106  391 

of  the  ring  finger,  as  above,  two  touches  may  be  simultaneously  applied, 
one  to  the  radial  side  of  the  middle  finger,  the  other  to  the  nlnar 
side  of  the  ring  finger.  Or  the  distances  may  be  compared  when  two 
compass  points  are  simultaneously  applied  one  to  each  of  the  crossed 
fingers,  the  points  first  being  close  together,  and  secondly  much 
wider  apart.  Or,  again,  the  compass  points  may  be  applied  diagonally 
(instead  of,  as  in  the  previous  experiment,  transversely)  across  the  two 
finger-tips,  and  the  subject  be  asked  in  which  direction  the  diagonal 
lies.  Greater  pressure  may  be  made  on  one  of  the  two  points, 
the  subject  being  asked  to  state  which  finger  is  being  pressed 
upon. 

The  results  differ  for  different  individuals,  some  of  the  illusions 
being  present  in  certain  people  which  are  not  obtainable  in  others. 

Another  striking  variation  of  the  experiment  is  as  follows. 
Holding  the  compass  points  so  that  the  line  joining  them  is  vertical, 
the  experimenter  applies  them,  one  to  the  upper,  one  to  the  lower 
lip  of  the  subject — (i.)  when  the  lips  are  in  the  normal  position,  (ii.) 
when  they  are  laterally  displaced  from  each  other.  The  subject 
estimates  the  inclination  of  the  imaginary  line  between  the  compass 
points. 

ABSOLUTE  LOCALISATION. 

Bxp.  106.  The  experimenter  makes  a  "  life  size  "  outline  sketch  of 
the  flexor  surface  of  the  subject's  left  forearm,  as  it  rests  comfortably 
upon  a  table.  He  draws  in  such  veins  and  tendons  as  may  serve  as 
landmarks.  The  subject  is  blindfolded.  His  right  hand  hangs  by 
his  side,  holding  a  blunt-pointed  wooden  rod.  After  a  warning  signal, 
the  experimenter  touches  the  subject's  left  forearm  with  a  similar  rod. 
He  removes  it  after  two  seconds,  whereupon  the  subject,  still  blind- 
fold, brings  his  own  rod  as  precisely  as  possible  to  the  same  spot. 
The  experimenter  carefully  measures  the  distance  between  the  two 
touches,  and  records  the  position  of  each  in  the  diagram,  joining  the 
two  by  a  line  which  bears  an  arrow  indicating  the  direction  of  the 
error,  while  the  length  of  the  line  indicates  the  extent  of  the  error  of 
localisation. 

Several  such  tests  are  made  on  various  parts  of  the  subject's 
forearm,  the  experimenter  taking  care  that  the  conditions  (e.g.  the 
duration  of  his  touch,  the  amount  of  pressure  exerted  by  it,  and  the 
interval  between  the  two  touches)  remain  as  constant  as  possible. 
The  order  of  the  successive  tests  should  be  noted  on  the  diagram,  and 
at  the  end  of  the  series  the  records  should  be  investigated  with  the 
object  of  showing  any  general  tendency  of  error,  and  the  influence  of 
practice  and  of  the  position  of  the  touch.  The  experimenter  must 


392  EXPERIMENTAL  PSYCHOLOGY 

always  be  on  the  look-out  for  anything  in  the  behaviour  of  the  subject 
which  may  throw  light  on  the  attitude  of  the  subject  during  localisa- 
tion. The  latter  makes  careful  introspective  observations  throughout 
the  experiment,  with  the  same  object  of  revealing  the  psychological 
factors  involved  in  his  acts  of  localisation. 

The  influence  of  a  comparison  of  the  two  touches  by  the  subject 
may  be  eliminated  by  forbidding  him  to  move  his  rod  after  he  has 
once  touched  his  skin.  The  influence  of  visual  imagery  may  be 
heightened  by  requiring  the  subject  to  open  his  eyes  and  apply 
his  rod  to  a  plaster  cast  or  to  a  photograph  of  his  arm, 
instead  of  to  his  own  arm,  or  by  asking  him  in  one  series  of  the 
previous  experiments  to  attend  solely  to  his  tactual  experience,  and 
in  another  series  solely  to  his  visual  imagery.  Visual  imagery  may 
be  entirely  dissociated  from  tactual  experience,  if  a  previously  marked 
spot  on  his  arm  be  shown  to  the  subject,  who  thereupon  closes  his  eyes 
and  endeavours  to  touch  it.  Or  the  combined  influence  of  vision  and 
touch  may  be  further  investigated,  by  allowing  the  subject  to  see  the 
experimenter's  touch  before  he  closes  his  eyes  and  attempts  to 
localise  it. 

The  influence  of  visual  factors  on  tactual  localisation  ir  so  great 
as  to  increase  the  accuracy  of  the  act  considerably.  It  is  often  shown 
in  the  tendency  of  the  subject  to  displace  the  touch  towards  service- 
able visual  landmarks,  e.g.  bony  prominences,  the  margins  of  the 
arm,  the  flexion  folds  of  the  skin. 

THE  VISUAL  PERCEPTION  OF  MOVEMENT. 

Exp.  107.  The  subject  makes  a  series  of  vertical  marks  along  the 
smoked  surface  of  a  drum  rotating  on  a  vertical  axis.  He  observes 
these  marks  for  fifteen  seconds  during  rotation.  He  then  stops  the 
drum  and  continues  to  fixate  one  of  the  marks  on  the  smoked 
surface,  observing  the  after-effects  of  movement. 

Exp.  108.  The  subject  fixates  a  horizontally  striped  black  and 
white  background,  a  central  vertical  strip  of  which  can  be  set  in 
continuous  motion.  He  observes  the  apparent  movement  of  the 
really  fixed  parts  of  the  ground,  which  is  set  up  during  actual  move- 
ment of  the  central  strip,  and  the  general  reversal  of  movement 
occurring  when  the  central  strip  is  brought  to  rest. 

Exp.  109.  The  subject  observes  the  after-effects,  obtained  after 
rotating  on  the  colour  wheel  a  white  disc,  on  which  a  broad  black 
spiral  figure  has  been  traced.  The  after-effects  should  be  noted — 


EXPERIMENTS  110,  111  393 

(i.)  when  fixation  is  still  confined  to  the  now  resting  disc,  and  (ii.)  when 
it  is  transferred  to  other  objects. 

The  subject  now  conceals  one-half  of  the  rotating  disc  by  a  white 
screen,  and  fixates  a  point  on  its  edge,  so  that  part  of  the  retina 
receives  the  moving  image  of  the  half-spiral,  and  the  other  part  receives 
the  image  of  the  stationary,  screen.  He  observes  the  after-effect  when 
the  rotation  is  stopped. 

Exp.  110.  The  subject  draws  a  straight  line  ABODE  (fig.  48) 
and  divides  it  into  imaginary  quarters.  He  then  draws  a  compass 
point  slowly  along  the  imaginary  arc  F  B  G  D  H.  Fixing  his  regard 


FIG.  48. 

steadily  on  the  moving  point,  he  observes  what  apparent  changes 
take  place  in  the  direction  of  the  line  A  C  as  the  point  moves 
from  F  to  G,  and  in  the  direction  of  the  line  C  E  as  the  point 
moves  from  G  to  H. 


EXERCISES  ON  CHAPTER  XVIII 

Sensibility  and  Sensory  Acuity 

VISUAL  ACUITY. 

Exp.  111.  The  determination  is  best  made  out  of  doors  on  a  dull 
cloudy  day.  For  the  E  method  either  Snellen's  illiterate  test-types 
(fig.  49)  or  Cohn's  arrangement  (fig.  50)  may  be  employed.  The 
following  applies  to  the  use  of  the  test  types. 

One  eye  of  the  subject  is  covered,  while  the  other  is  tested.  He 
stands  at  a  distance  of  15  metres  from  the  letters.  The  experimenter 
points  to  the  largest  letters  individually  ;  the  subject  is  asked  to  place 
in  a  corresponding  position  the  E  with  which  he  is  provided,  or  merely 
to  state  whether  the  open  ends  of  the  E  face  right,  left,  up  or  down- 
wards. The  experimenter  proceeds  from  larger  to  smaller  letters 
until  he  reaches  a  line  in  which  two  mistakes  in  ten  replies  occur. 
This  is  an  arbitrary  limit,  but  serves  satisfactorily. 


394 


EXPERIMENTAL  PSYCHOLOGY 


The  subject  is  now  moved  a  metre  nearer  to  the  letters,  and  the 
test  is  resumed.     He  advances  in  this  way  by  successive  metres  until 
_        _  he  can  read  the  smallest  type,  known  as  No.  5,  with- 

I  I  I  out  making  niore  than  two  mistakes  in  ten  answers. 

^J  The  visual  acuity  is  expressed  as  a  fraction,  the 

numerator  denoting  the  distance  in  metres  at  which 
the  subject  can  just  read  a  letter,  the  size  of  which 
is  the  denominator.  The  size  of  a  letter  is  denoted 
by  the  number  of  metres'  distance  at  which  that 
letter  should  be  read  by  persons  possessing  so-called 
"  normal  "  acuity.  For  such  persons  the  numerator 
and  denominator  have  the  same  value. 

The  subject  must  make  careful  introspection  of 
'    IE!    111    PI    kis  mental  procedure,  and  of  any  alterations  occur- 
ring therein  during  the  investigation. 

If  opportunity  permit,  he  should  compare  the 
J  3  E  PI  HI      numerical  and    introspective  data    thus    obtained 
with  those  obtained  by  the  use  of  such  other  tests 
u  m  E  uj  a  E         of  visual  acuity  as  the  laboratory  possesses. 


3   HI 

E  111  3 


E  3  m  m  u  E  u 

FIG.  49. 


AUDITORY  ACUITY. 
Bxp.    112.  The   determination  is  best  made 


out  of  doors  on  a  still,  windless  night. 
Politzer's  acoumeter  (fig.  51)  serves  as  a 
convenient  source  of  sound.  The  subject 
turns  one  ear  towards  this  instrument, 
which  is  held  between  the  experimenter's 
fingers,  at  a  distance  of  five  metres.  The 
subject's  other  ear  should  be  lightly  stopped 
with  cotton  wool.  The  small  percussion 
hammer  of  the  acoumeter  is  allowed  by 
the  experimenter  to  fall  five  times  at 
irregular  intervals.  He  utters  a  warning 
"  Now  ! "  before  the  first  fall,  and  the  FlQ  50  _  Tlie  i^ter  E  in 
subject  exclaims  "  Yes  "  or  "  No,"  as  he  various  positions  is  drawn  in 
hears  or  fails  to  hear  the  sound.  The 
subject  is  forewarned  that  sometimes  no 
sound  will  be  given,  and  the  experi- 
menter takes  care  irregularly  to  interpose 
five  such  catch  experiments  among  every 
five  sounds. 

The   experimenter  increases   his    dis- 


black  on  a  white  card,  as 
in  the  figure.  This  card  is 
covered  by  a  cardboard  disc, 
in  which  a  circular  window 
is  cut.  By  rotating  the  disc, 
the  experimenter  can  exhibit 
to  the  subject  any  E  he  de- 


EXPERIMENT  113  395 

tance  from  the  subject  by  a  metre,  and  sounds  the  instrument  five 
times,  and  gives  five  catch  experiments  as  before.  The  experimenter 
is  careful  to  record  all  the  subject's  answers. 
He  continues  to  withdraw  the  instrument 
metre  by  metre  in  this  way,  until  the  sub- 
ject fails  to  hear  one  of  the  five  sounds. 
The  experimenter  then  gives  another  series 
of  five  sounds  and  five  "  catches,"  at  the 
same  distance ;  and  if  another  sound  is 
missed,  this  distance  may  be  arbitrarily  re-  Y 

garded  as  the  threshold.  ^'^'^  ^T*  f 

_       "    '  held  between  the  thumb 

The  experimenter,  however,  should  in-      and  second  finger  at  x  and 

crease  his  distance  still  farther  several  times,  y.     The  lever   L   is  de- 

and    observe    the    resulting    replies.      He  pressed  by  the  forefinger 

should  then  gradually  bring  the  instrument  until  it  touches  the  cork  C, 

nearer  to  the  subject  by  the  same  stages,  and  then  tt  is  suddenly  re- 

and  observe  the  distance  at  which  not  more  leaf*'  By  t^eaos  the 
,.  ,  .  ,  ,  i.  metal  hammer  H  is  allowed 

than  two  sounds  in  ten  are  missed  by  the     to  fall  freely  from  a  con. 

subj  ect.  stant  height  on  to  the  bar  B. 

Throughout  the  experiment,  the  subject 

carefully  observes  his  experiences,  and  at  the  close  writes  a  full 
account  of  his  introspection.  The  subject  and  experimenter  then 
endeavour  to  correlate  the  results  of  introspection  with  the  answers 
given,  paying  due  regard  to  the  illusions  and  irregularities  in  the 
subject's  answers. 

OLFACTORY  ACUITY. 

Bxp.  113.  The  experimenter  dissolves  a  gram  of  camphor  in 
1000  c.c.  of  odourless  distilled  or  rain  water.  He  prepares  from  this 
a  series  of  camphor  solutions,  of  strengths  1  : 4000,  1 :  8000,  1  : 16,000, 
1  : 32,000,  etc.  A  number  of  cylindrical  glass  tubes,  closed  at  one 
end,  must  be  in  readiness.  They  should  measure  about  75  mm.  in 
height  and  25  mm.  in  width,  and  be  scrupulously  clean  and  free 
from  smell. 

The  experimenter  first  attempts  to  arrive  at  an  extremely  rough 
determination  of  the  olfactory  acuity  of  the  subject,  by  asking  him  to 
smell  the  variously  diluted  camphor  solutions  successively.  Only  one 
or  two  minutes  should  be  given  to  this  part  of  the  experiment,  in 
order  to  avoid  the  onset  of  fatigue.  The  solutions  are,  therefore, 
rapidly  sniffed  in  succession,  until  a  solution  is  reached  in  which  no 
odour  of  camphor  can  be  detected. 

The  experimenter  begins  with  a  camphor  solution  which  is  a  stage 
stronger  than  that  which  appears  to  be  roughly  liminal.  While  the 


396  EXPERIMENTAL  PSYCHOLOGY 

subject  is  resting  (in  order  that  he  may  recover  as  completely  as 
possible  from  the  just-mentioned  procedure),  the  experimenter  takes 
four  of  the  cylindrical  glass  vessels,  two  of  which  are  to  contain  50  c.c. 
of  odourless  water,  and  two  the  same  amount  of  the  solution  of 
camphor.  The  vessels  are  marked,  preferably  on  their  base,  so  that 
the  contents  of  each  are  identifiable  only  by  the  experimenter.  He 
now  prepares  a  scheme  of  five  different  orders  in  which  the  four 
vessels  are  to  be  successively  presented  to  the  subject.  It  is  well  to  pre- 
pare two  such  schemes,  and  to  use  sometimes  one,  and  sometimes  the 
other,  so  that  the  subject  cannot  possibly  become  acquainted  with  the 
orders. 

An  interval  of  fifteen  minutes  having  elapsed,  the  four  tubes  are 
now  set  before  the  subject,  who  bends  over  and  smells  them  successively. 
He  is  not  allowed  to  touch  them,  or  to  return  to  a  solution  after  he 
has  given  a  reply.  He  is  to  answer  "  Camphor  "  or  "  Water,"  before 
he  proceeds  from  one  solution  to  smell  the  next.  The  subject  then 
turns  his  back,  while  the  experimenter  changes  the  order  of  the  same 
four  vessels,  in  accordance  with  his  scheme.  The  subject  smells  them 
again.  The  order  is  then  changed,  and  they  are  smelled  again,  the 
procedure  being  repeated  until  the  twenty  replies  are  recorded. 
Record  is  carefully  kept  of  all  answers,  and  an  arbitrary  threshold  is 
fixed  which  allows  two  wrong  answers  in  ten  in  respect  of  the  camphor 
solutions. 

If,  as  is  probable,  the  limit  has  not  been  reached,  the  experiments 
are  continued  by  substituting  a  weaker  solution  of  camphor  in  the 
next  series. 

A  pause  of  ten  minutes  should  be  allowed  in  passing  from  one 
series  of  twenty  answers  to  the  next. 

Occasionally  a  fifth  vessel  containing  either  camphor  solution 
or  water,  or  instead  of  four  only  three  vessels,  may  be  exposed,  in 
order  to  avoid  certain  inferences  on  the  part  of  the  subject;  but  the 
cautions  mentioned  on  page  209  must  be  borne  in  mind. 

The  experimenter  should  gradually  pass  to  a  solution  the  strength 
of  which  is  definitely  below  that  required  for  the  arbitrary  threshold. 
He  should  then  reverse  the  previous  procedure  and  present  increasing 
strengths  of  the  solution  until  the  threshold  is  once  again  reached  and 
overstepped. 

VISUAL  ADAPTATION. 

Bxp.  114.  The  effects  of  dark  adaptation  (exp.  69),  and  of  adapta- 
tion to  colourless  and  colour  stimuli  (exp.  74)  have  been  already 
demonstrated.  The  student  may  investigate  the  effects  of  wearing 
coloured  glasses  for  some  time. 


EXPERIMENTS  115-117  397 

PURKINJE'S  IMAGES. 

Exp.  115.  The  experimenter  and  the  subject  are  in  a  dark  room. 
The  former  concentrates  the  light  of  a  candle  by  means  of  a  double 
convex  lens  of  short  focus  on  to  the  outer  (temporal)  corner  of  the 
subject's  sclerotic,  who  turns  his  eye  inwards,  regarding  preferably  a 
light- coloured  patternless  wall.  The  subject  will  soon  see  the  shadows 
of  the  retinal  vessels  as  a  dark  arborescence  on  a  yellowish-red  field. 
Under  ordinary  conditions  these  vessels  are  invisible,  mainly  owing 
to  sensory  adaptation. 

THE  BLIND  SPOT. 

Exp.  116.  The  existence  of  the  blind  spot,  the  point  of  entrance 
of  the  optic  nerve,  may  be  conveniently  demonstrated  here,  although 
the  filling  out  of  this  spot  (which  occurs  under  the  ordinary  conditions 
of  daily  life)  is  not  exactly  an  instance  of  adaptation.  Although  the 
spot  is  devoid  of  rods  and  cones,  the  subject  irresistibly  supplies  the 


O 


FIG.  52. 


sensations  which  are  actually  wanting  there,  being  guided  by  the 
mode  in  which  neighbouring  sensitive  regions  of  the  retina  are  being 
simultaneously  stimulated.  A  simple  method  of  demonstrating  the 
blind  spot  consists  in  closing  one  eye  (the  left)  while  the  other  is 
fixed  on  a  point  marked  on  a  card  (fig.  52).  At  some  distance  to 
one  side  of  the  point  the  card  bears  a  circle,  which  disappears  when 
the  card  is  moved  to  such  a  distance  from  the  eye  that  while  the 
image  of  the  point  falls  on  the-fovea,  that  of  the  circle  falls  on  the 
blind  spot. 

AUDITORY  ADAPTATION. 

Exp.  117.  The  subject  places  the  ends  of  a  rubber  tube  one  in 
each  ear,  and  sits  before  a  table  on  which  the  tube  rests.  The  experi- 
menter gently  rests  a  vibrating  tuning-fork  on  the  midpoint  of  the 
tube,  compressing  the  latter  to  one  side  so  that  the  tone  is  conducted 
only  to  one  ear.  As  soon  as  it  ceases  to  be  audible  to  the  subject,  the 
observer  gently  releases  the  tube  on  the  other  side  of  the  fork,  so  that 
the  hitherto  unused  path  is  now  available. 


398  EXPERIMENTAL  PSYCHOLOGY 

Exp.  118.  The  subject  places  a  vibrating  fork  on  the  vertex  of 
his  head  and  retains  it  until  the  tone  appears  to  have  died  away.  Then 
he  removes  it  only  for  an  instant  and  places  it  as  before  on  the  head. 

If  in  these  two  experiments  the  tone  is  again  heard  after  it  had 
disappeared,  the  student  should  consider  the  various  factors  on  which 
the  phenomenon  may  depend. 


EXEECISES  ON  CHAPTER  XIX 

Experiences  of  Identity  and  Difference 

WEBER'S  LAW. 

Exp.  119.  The  experimenter  and  subject  should  perform  a  series 
of  experiments  with  weights,  using  the  limiting  method  described  on 
page  205. 

THE  LEAST  PERCEPTIBLE  DIFFERENCE  OF  PITCH. 

Exp.  120.  Probably  tuning-forks  will  be  the  most  convenient 
instruments  for  the  student's  use.  No  source  of  sound,  however,  is 
entirely  free  from  disadvantages  ;  there  is  varying  difficulty  in  securing 
uniformity  of  loudness,  pitch  and  timbre.  The  method  of  serial  groups 
(page  209)  may  be  recommended  for  the  elementary  student. 

The  influence  of  time  order  (standard  presented  first  or  second) 
should  be  studied.  Full  introspective  records  should  be  obtained 
from  the  subject,  with  regard  to  imagery,  tendencies  to  movement  of 
the  glottis,  and  his  modes  of  arriving  at  a  judgment. 

THE  ABSOLUTE  IMPRESSION. 

Exp.  121.  The  experiment  described  on  pages  268-272  should  be 
performed  in  the  laboratory.  A  standard  weight,  S,  of  400  grams, 
and  four  variable  weights,  S  +  d,  and  $  +  2(2,  may  be  used,  d  being 
20  grams.  The  answers  should  be  scheduled  as  follows  : 

Order  S-2d  S-d  S  +  d  S+2d 

I 
2 
3 
4 

Here  the  percentage  of  correct  judgments,  when  S  is  lifted  with 
S—2d  and  S— d,  corresponds  to  the  a  judgments,  and  the  percentage 
of  correct  judgments,  when  Sis  lifted  with  S+d  and  S+2d,  corresponds 
to  the  b  judgments  of  the  text.  The  four  orders  in  the  schedule  refer 


EXPERIMENT  122  399 

to  the  possible  combinations  of  different  temporal  and  spatial  orders 
(e.g.,  a1}  «2  >^3  5^4)  mentioned  in  the  text. 

The  variables  should  be  used  with  the  standard  in  quite  haphazard 
order,  until  (say)  each  variable  has  been  presented  five  times  in  each 
of  the  four  orders,  that  is  until  80  judgments  have  been  recorded. 
"Doubtful"  and  "equal"  judgments  may  be  divided  equally  between 
the  right  and  wrong  judgments  (page  214). 

Eighty  answers  will  provide  material  for  the  student  to  elucidate 
the  influence  of  the  absolute  impression,  the  general  tendency  of 
judgment  and  the  effects  of  time  and  space  order.  But  no  confidence 
can  be  placed  in  the  results  of  so  few  observations. 

EQUAL-APPEARING  INTERVALS  OF  BRIGHTNESS. 

Exp.  122.  Two  "standard"  grey  papers,  a  and  6,  of  different 
brightness  are  presented  upon  a  black  or  white  ground.  Accompany- 
ing them  is  a  third  paper,  the  variable  c,  one  of  a  series  of  various 
greys.  By  one  of  the  recognised  psycho-physical  methods  the  experi- 
menter has  to  find  a  paper  c  of  such  brightness  that  to  the  subject  the 
difference  between  a  and  b  appears  equal  to  that  between  6  and  c.  The 
papers  must  be  of  exactly  the  same  size  and  uniformly  illuminated. 
The  light  values  of  the  various  grey  papers  having  been  determined 
as  described  below,  the  relation  of  the  intensities  of  the  stimuli  a,,  6,  c 
to  the  sensation  differences  can  be  investigated.  The  chief  causes  of 
departure  from  Weber's  law  have  been  already  sufficiently  discussed 
in  the  text. 

The  light  values  of  the  grey  papers  may  be  determined  in  the 
following  manner.  It  may  be  assumed  that  the  greatest  value  of 
reflected  light  obtainable  from  the  surface  of  a  paper  is  afforded  by 
a  sheet  of  baryta-covered  paper.  This  will  serve  as  the  standard,  and 
will  be  given  the  value  of  360.  The  light  values  of  other  white,  grey, 
and  black  papers  may  be  expressed  in  terms  of  the  standard,  a  paper 
yielding  half  the  light  of  the  standard  receiving  the  value  180,  and  so 
on.  It  may  be  further  assumed  that  an  absolutely  black  background 
is  obtainable  by  looking  into  a  long  tube,  say  75  cm.  long  and  about 
30  cm.  in  diameter,  closed  save  for  a  small  opening,  and  lined  with  black 
velvet.  Before  this  opening  a  colour  wheel  is  set  up,  and  on  the  colour 
wheel  is  rotated  a  smaller  disc  of  the  paper  to  be  tested  and  a  project- 
ing sector  of  larger  radius,  of  the  standard  paper.  This  sector  being 
rotated  with  sufficient  speed  before  the  black  aperture,  flicker  is 
finally  abolished,  and  an  outer  ring  is  obtained  which  is  comparable 
with  the  smaller  disc  on  the  colour  wheel.  The  arc  of  the  sector 
is  increased  or  diminished  until  the  two  colourless  surfaces  are  identi- 


400  EXPERIMENTAL  PSYCHOLOGY 

cal.  The  light  value  of  the  paper  to  be  tested  is  then  given  by  the 
number  of  degrees  covered  by  the  arc.  Of  course,  any  white  paper 
may  be  used  as,  and  in  place  of,  the  standard,  if  it  have  previously 
been  standardised  with  the  latter. 

The  most  convenient  manner  of  arranging  the  papers  on  the  colour 
wheel  is  here  indicated  (fig.  53).  A  represents  the  diameter  of  the 
smaller  disc,  the  paper  to  be  tested.  B  represents  on  the  same  scale 
the  standard  paper  which  is  placed  behind  A  on  the  colour  wheel. 
The  size  of  the  projecting  sector  is  so  chosen  that  on  rotation  of  the 
wheel  the  outer  ring  is  darker  than  the  inner  disc.  Behind  B  is 
placed  a  second  piece  of  the  standard  paper  B'  precisely  similar  to 
it.  Then  B  and  B'  can 
be  accurately  super- 
imposed, or  B'  can  be 
turned  round  so  that 
more  of  its  segment 
conies  to  project  be- 
yond the  segment  of  B, 
until  upon  rotation  of 
the  wheel  the  desired 
match  is  obtained.  FIG.  53. 

When  once  the  light 

value  of  a  second  paper  has  been  estimated,  the  value  of  any  third 
paper  may  be  readily  found  without  the  use  of  a  dark  chamber  by 
rotating  a  disc  of  the  latter  paper  on  the  colour  wheel,  while  behind 
it  are  arranged  two  larger  discs  of  the  already  standardised  papers, 
slit  from  centre  to  periphery  in  the  ordinary  fashion,  and  movable 
one  over  the  other,  until  on  rotation  the  outer  ring  and  inner  disc 
match  one  another.  If,  for  example,  the  match  requires  125°  of  the 
standard  paper  and  235°  of  the  already  standardised  grey  paper,  and 
if  their  light  values  be  360  and  75  respectively,  then  the  light  value 

of  the  paper  in  the  inner  ring  is  125H — = —  =  173'96  nearly. 

ouO 


EXEECISES  ON  CHAPTEE  XX 

Binocular  Experience 

BINOCULAR  COMBINATION. 

Bxp.  123.  Two  similar  shillings  are  placed,  about  15  cm.  apart, 
on  a  glass  plate  which  is  held  close  to  the  mid-line  of  the  body.    The 


EXPERIMENTS 


401 


image  of  a  third  single  intermediate  shilling  can  now  be  obtained  by 
binocular  combination,  the  eyes  fixating  a  pencil  point  which  is  held 
(i.)  nearer  to  or  (ii.)  farther  from  the  eyes  than  the  two  coins.  The 
glass  plate  should  be  moved  towards  or  away  from  the  fixation  point 
until  binocular  combination  is  effected.  Attention  should  be  drawn 
to  any  differences  in  the  ease  of  combination  and  in  the  size  or 
localisation  of  the  combined  image,  according  as  the  coins  lie  nearer 
to  or  further  from  the  eyes  than  the  point  of  fixation. 

Bxp.  124.  The  student  should  familiarise  himself  with  the  theory 
and  use  of  Wheatstone's  mirror  stereoscope  and  Brewster's  prism 


X 

J 

r\ 

\ 

M' 

V 

/       \ 

M" 

\ 

J            \, 

/ 

A    ;     \    B 


\/ 


6 


FIG.  54. 


stereoscope  (fig.  54).  Owing  to  the  action  of  the  mirrors,  M'  M",  or  of 
the  prisms,  P'  P",  the  objects  A  and  B  are  combined  by  the  two  eyes, 
E'  E",  and  referred  to  X. 

DIPLOPIA. 

Bxp.  125.  The  image  of  a  single  shilling  placed  as  before  on  a 
glass  plate,  may  be  doubled,  the  fixation  point  (of  a  pencil)  being 
nearer  to  or  further  than  the  coin.  The  student  should  observe  the 
different  effect  produced  upon  the  doubled  image  in  the  two  cases  by 
closing  one  eye,  and  correlate  these  differences  with  the  disparation 
(page  274). 

THE  CYCLOPEAN  EYE. 

Bxp.  126.  A  piece  of  paper  is  held  horizontally  before  the  eyes, 
on  which  two  parallel  lines  have  been  drawn,  separated  by  a  distance 
26 


402 


EXPERIMENTAL  PSYCHOLOGY 


equal  to  that  between  pairs  of  corresponding  points.  When  the  gaze, 
travelling  over  the  two  lines  along  the  surface  of  the  paper,  is  directed 
to  a  distant  point,  only  a  single  line  will  be  seen,  situated  midway 
between  the  two  eves. 


FAILURE  TO  IDENTIFY  THE  EYE  STIMULATED. 

Exp.  127.  The  experimenter  takes  a  large  sheet  of  black  cardboard, 
pierced  with  a  minute  aperture,  and  he  moves  the  card  continuously 
but  irregularly  in  front  of  the  observer's  face,  so  that  light  is  admitted 
through  the  aperture,  now  to  one,  now  to  the  other  eye.  The  observer 
will  note  that,  after  these  movements  have  been  carried  on  for  a  brief 
time,  he  is  unable  to  tell  which  eye  is  receiving  light  when  the 
experimenter  ceases  to  move  the  card. 

DEPTH  AND  EETINAL  DISPARATION. 

Exp.  128.  Using  the  apparatus  provided,  the  student  moves  the 
two  vertical  threads  A  and  C  (fig.  55)  to  such  an  extent  apart  that 
0  when  the  eyes  are  directed 

to  a  more  distant  point, 
on  the  opposite  wall,  the 
threads  (one  in  front  of 
each  eye)  yield  a  single 
combined  image.  Then 
he  arranges  another  pair 
of  vertical  threads  B  and 
D,  beside  and  in  the  same 
plane  with  the  former 
pair,  so  that  they  also 
give  rise  to  a  combined 
image.  He  observes  now 


the  effect  of  slightly  in- 
Fio.  55.  creasing  or  decreasing  the 

distance  between  B  and 

D,  so  that  the  retinal  points,  excited  by  B  and  D,  are  no  longer 
corresponding  but  disparate. 

BERING'S  FALL  EXPERIMENT. 

Exp.  129.  This  experiment  shows  the  difficulty  of  judging 
relative  distance  with  monocular  vision.  Applying  his  eye  to  the  end 
of  a  cardboard  tube,  the  subject  fixates  a  small  bead,  keeping  the  other 
eye  closed.  The  experimenter  drops  successive  beads  in  front  of  or 


EXPERIMENTS  130,  131 


403 


behind  the  point  of  fixation,  and  notes  the  correctness  or  incorrectness 
of  Hhe  judgments  given  by  the  subject.  A  series  of  observations  is 
then  made  when  both  eyes  are  open.  The  subject  should  at  the  same 
time  keep  an  introspective  record.  It  should  be  considered  whether 
the  apparatus  in  its  rough  form  is  altogether  free  from  objection. 

BINOCULAR  RIVALRY,  COMBINATION  AND  LUSTRE. 

Exp.  130.  The  effects  of  binocular  colour  rivalry,  combination 
and  lustre  should  be  studied  in  one  or  other  form  of  stereoscope  ; 
coloured  squares,  black  and  white  squares,  and  various  designs  and 
objects  being  used.  The  relation  of  rivalry  to  discrepancy  in  contour, 
intensity  or  brightness,  should  be 
observed. 

Hering  has  devised  an  arrange- 
ment for  showing  binocular  colour 
combination  and  rivalry,  in  which 
the  two  eyes  look  each  through  a 
differently  coloured  (e.g.  a  blue  and 
a  red)  glass  at  a  square  of  white  light. 

Figure  56  shows  this  arrange- 
ment. B  and  R  are  the  blue  and  red 
glasses,  fixed  with  their  edges  juxta- 
posed in  the  dark  box,  to  one  end 
of  which  the  eyes,  E1  and  E2,  are 
applied.  At  the  other  end  are  three 
squares  of  ground  glass,  6,  ??t,  and  r, 
through  which  the  observer  gazes  on 
to  a  uniform  white  surface,  e.g.  a 
clouded  sky. 

Under  these  conditions,  the  two  lateral  squares  are  coloured 
according  to  the  glass  placed  before  the  eye,  while  the  central  one 
shows  alternating  colour  combination  and  rivalry. 

BINOCULAR  CONTRAST. 

Exp.  131.  By  fixating  a  nearer  point,  the  student  produces 
double  images  of  a  white  stripe  on  a  black  background.  He  places  a 
red  glass  before  the  one  eye  and  an  equally  bright  grey  glass  before 
the  other.  He  observes  if  the  image  seen  through  the  grey  glass  is 
tinged  with  green  (the  complementary  colour  to  red).  He  then 
removes  the  red  glass  and  observes  that  the  image,  yielded  by  the  eye 
which  has  been  covered  with  the  grey  glass,  becomes  a  well-marked 
red. 


404  EXPERIMENTAL  PSYCHOLOGY 

Exp.  132.  The  light  of  a  window  is  allowed  to  fall  laterally  on 
one  eye,  while  the  other  is  consequently  more  shaded.  The  student 
now  doubles  the  image  of  a  white  stripe  on  a  black  background  by 
fixating  a  nearer  point.  He  observes  the  difference  in  brightness  and 
colour  of  the  two  images. 

This  experiment  is  called  Fechner's  side- window  experiment,  and 
is  dependent  on  the  exposure  of  one  eye  to  a  brighter  light  which 
enters  the  eye  through  the  sclerotic  and  iris,  acquiring  a  reddish  tinge 
owing  to  its  passage  through  a  layer  of  blood  vessels.  Ketinal  adap- 
tation and  binocular  contrast  afford  a  partial  explanation  of  the  effects. 

Exp.  133.  The  effects  of  uniocular  contrast  and  binocular  combina- 
tion are  observed  in  a  simple  apparatus  devised  by  Bering,  in  which 
a  black  stripe  on  a  white  ground  is  doubled  by  fixation  of  a  nearer 
point,  and  is  viewed  by  one  eye  through  a  red,  by  the  other  through 
a  blue  glass. 

BINOCULAR  BRIGHTNESS. 

Bxp.  134.  The  observer  places  a  moderately  dark  grey  glass 
before  one  eye  which  is  closed,  while  the  other  regards  a  white 
surface.  He  observes  the  brightness  of  the  latter,  and  he  compares  it 
with  its  brightness  when  the  shaded  eye  is  opened  and  the  surface  is 
regarded  binocularly. 

LISTING'S  LAW. 

Bxp.  135.  This  law  may  be  verified  by  projecting  the  after- 
image of  a  rectangular  cross  on  to  various  points  on  a  plane  surface. 
Figure  57  represents  a  conveniently  prepared  surface,  the  centre  of 
which  is  occupied  by  a  coloured  cross.  The  head  of  the  subject  is 
comfortably  fixed  so  that  it  cannot  move  when  the  eyes  are  turned. 
The  subject  is  seated  so  that  his  eyes,  when  regarding  the  cross,  are 
in  an  approximately  primary  position.  This  position  the  subject 
finds  by  fixating  the  cross  and  by  then  turning  the  eyes  to  one  or  other 
of  the  points  a,  6,  c,  d.  If  the  outline  of  the  various  after-images 
takes  the  direction  of  the  horizontal  and  vertical  ruled  lines  of  the 
surface,  the  primary  position  of  the  eyes  has  been  found.  If  the 
surface  can  be  suitably  rotated  round  its  centre,  the  corresponding 
condition  will  be  found  to  hold  good  for  oblique  as  for  horizontal  and 
vertical  positions  of  the  arms  of  the  cross. 

Let  the  eyes  be  obliquely  turned  so  as  to  project  the  after-image 
of  the  vertical  and  horizontal  arms  of  the  cross  on  to  the  points 
e,  /,  g  or  h.  The  unequal  displacements,  which  the  arms  of  the 
after-images  undergo,  are  shown  by  the  drawings  of  the  cross  at 


EXPERIMENT  136 


405 


these  points  in  figure.  But  were  the  surface  of  projection  a  large 
hemisphere  corresponding  to  the  curved  surface  of  the  retina,  instead 
of  being  a  plane,  these  dis- 
tortions would  not  occur, 
and  thus  Listing's  law 
would  be  verified.  The 
distortions  with  which  we 
meet  are  due  to  our  in- 
terpretation of  the  ruled 
lines  as  being  truly  hori- 
zontal and  vertical  in  spite 
of  the  fact  that  the  eyes 
are  now  regarding  them 
in  an  oblique  instead  of 
in  the  primary  position. 
Save  in  the  primary  posi- 
tion of  the  eyes,  horizontal 
and  vertical  lines  must 
really  give  rise  to  oblique 
retinal  images.  But  since 
these  lines  are  interpreted  as  being  free  from  obliquity,  a  correspond- 
ing obliquity  is  transferred  to  the  actually  horizontal  and  vertical  bars 
which  form  the  after-image  of  the  cross. 


FIG.  57. 


EXEKCISES  ON  CHAPTEK  XXI 

Binaural  Experience 

INFLUENCE  OF  BINAURAL  DIFFERENCES  OF  INTENSITY  AND  TIMBRE. 

Exp.  136.  The  subject  is  seated  blindfold  in  a  chair,  his  head 
supported  by  a  rest.  If  a  "  sound  perimeter  "  be  available,  the  experi- 
menter can  systematically  investigate  the  frequency  and  nature  of 
the  errors  in  localisation  (i.)  according  to  the  direction  of  the  sound, 
(ii.)  according  to  the  nature  of  the  sound,  and  (iii.)  according  to  the 
practice  of  the  subject. 

A  sound  perimeter  consists  of  a  graduated  metal  framework, 
supporting  the  source  of  sound  and  permitting  the  latter  to 
be  noiselessly  moved  in  various  directions  relatively  to  the 
subject. 

The  "  buzzer  "  of  a  telephone,  carried  on  the  perimeter,  will  serve 
as  a  complex  sound  stimulus;  an  electrically  driven  tuning-fork, 


406  EXPERIMENTAL  PSYCHOLOGY 

driven  by  a  distant  electrically  driven  fork  and  battery,  will  serve  as 
a  purer  tone  stimulus. 

The  experimenter  divides  the  horizontal  and  sagittal  planes  of 
space  in  the  following  manner.  The  subject  is  supposed  to  be  seated 
in  an  imaginary  sphere  the  centre  of  which  lies  midway  between  his 
ears.  The  two  points  on  the  mid-horizontal  plane  of  the  sphere, 
which  mark  the  poles  in  front  of  and  behind  him,  are  regarded  as 
0°  and  180°  respectively ;  the  two  points  lying  to  the  extreme  right 
and  left  of  him  are  regarded  as  90°  and  270°  respectively.  The 
sagittal  plane  is  similarly  divided,  90°  being  the  position  of  a  point 
above  the  vertex  of  the  subject. 

In  the  absence  of  a  perimeter,  the  accuracy  of  auditory  localisa- 
tion may  be  roughly  investigated  by  four  experimenters  standing 
respectively  in  front  of,  behind  and  to  each  side  of  the  subject,  upon 
a  graduated  chalk  circle  about  two  metres  in  diameter,  drawn  on  the 
floor.  Each  experimenter  holds  between  thumb  and  forefinger  two 
coins,  which,  when  clicked,  serve  as  the  source  of  sound.  One  of 
the  experimenters  directs  the  movements  of  the  others,  noiselessly 
indicating  to  one  or  other  of  his  colleagues  that  he  is  to  give  the 
sound  at  any  point  within  his  own  quadrant. 

In  the  absence  of  an  electrically  vibrating  tuning-fork,  two 
ordinary  forks  of  identical  pitch  may  be  employed,  held  by  separate 
experimenters  who  stand  one  on  each  side  of  the  subject.  Both  forks 
are  struck  simultaneously  by  preconcerted  signal;  but,  by  pre- 
arrangement,  one  of  them  is  damped  immediately  after  being  struck. 
The  subject  states,  as  before,  the  direction  from  which  he  supposes 
the  sound  to  come.  The  object  of  striking  two  forks  is  to  limit  the 
basis  of  the  subject's  answers  to  sensations  of  tone  and  to  exclude 
those  of  noise. 

The  blindfold  subject  verbally  describes  the  direction  from  which 
the  sound  appears  to  come  to  him,  and  one  of  the  experimenters 
carefully  records  the  actual  and  the  apparent  direction  of  the  sound. 
The  following  results  may  be  expected  : — 

(1)  Fairly  accurate  localisation  in  the  horizontal  plane. 

(2)  Gross  errors  of  localisation   in   the  vertical  sagittal  plane, 
especially  for  pure  tones,  diminishing  on  practice. 

(3)  Tendency  to  confuse  sounds  in  the  horizontal  plane  which  lie 
symmetrically  with  regard  to  the  transverse  (coronal)  plane,  e.g.  to 
interpret  a  sound  at  45°  as  coming  from  135°. 

The  subject  should  endeavour  to  examine  introspectively  the  basis 
of  his  several  judgments,  and  from  time  to  time  he  should,  if  possible, 
give  the  results  of  such  introspections  which  are  to  be  recorded  by 
the  experimenter  beside  the  subject's  estimations.  If  he  finds  it  too 


EXPERIMENTS  137-139  407 

difficult  to  attend  simultaneously  to  the  act  of  localisation  and  to  the 
mode  of  localisation,  a  series  of  experiments  may  be  subsequently 
conducted,  in  which  his  attention  is  more  completely  concentrated 
on  the  introspective  aspect  of  the  records,  even  at  the  expense  of  loss 
of  accuracy  of  localisation. 

At  the  same  time,  the  experimenter  should  be  on  the  look-out  for 
peculiarities  in  the  behaviour  of  the  subject  which  may  throw  light 
on  the  psychological  basis  of  his  power  of  localisation. 

Exp.  137.  Two  vibrating  forks  are  placed  one  on  each  side  of 
the  subject.  If  the  forks  are  in  unison,  and  affect  the  two  ears 
equally,  the  sound  is  localised  in  the  middle  line. 

INFLUENCE  OF  BINAURAL  DIFFERENCES  OF  PHASE. 

Exp.  138.  If  the  forks  give  beats  with  one  another,  the  sound 
will  be  alternately  localised  at  different  ears.  Now,  in  the  cycle 
between  any  two  beats,  the  differences  in  phase  between  the  two 
ears  assume  every  possible  value.  Supposing  that  the  right  hand 
fork  is  the  higher,  the  right  hand  effect  will  be  found  to  follow 
binaural  agreement  in  phase,  the  left  hand  effect  to  follow  opposition 
in  phase. 

It  will  also  be  found  that,  as  the  forks  approach  the  ears,  the 
localisation  becomes  intra- cranial,  i.e.  the  sound  is  heard  within 
the  head. 

Exp.  139.  The  subject  places  the  two  ends  of  a  long  tube  one  in 
each  ear,  and  closes  his  eyes,  the  tube  resting  on  a  table.  The 


experimenter  lightly  sets  a  vibrating  tuning-fork  on  the  tube,  along 
which  he  moves  it  now  in  one  direction,  now  in  another.     The 


4o8  EXPERIMENTAL  PSYCHOLOGY 

subject  localises  the  tone  intra-cranially,  the  tone  wandering  within 
the  head  from  one  ear  to  another,  according  to  the  direction  of 
binaural  difference  in  intensity  or  wave  length. 

The  arrangement  of  apparatus,  shown  in  figure  58,  enables 
the  apparent  influence  of  binaural  phase  difference  upon  localisation 
(page  287)  to  be  more  strikingly  shown.  Here  the  tuning-fork  K 
is  applied  to  the  open  end  of  the  T-piece,  T,  the  long  limb  of  which 
is  a  brass  tube  AB,  sliding  within  the  slightly  larger  tubes  AC,  BF. 
The  opening  T  can  be  brought  to  any  position  of  the  scale  DE, 
between  40  and  160  cm.  H  is  the  head  of  the  subject,  whose  view 
of  the  position  of  T  is  prevented  by  the  screen  SS.  Against  his 
head  are  pressed  the  two  padded  ear  caps,  P  and  Q,  which,  supported 
on  retort  stands  M  and  N,  receive  the  sound  from  the  tubes  AC,  BF. 
If  a  be  the  distance  of  T  from  the  centre  of  the  scale,  and  if  X  be  the 
wave  length  of  the  tone,  the  tone  is  localised  on  the  (experimenter's) 

right  of  the  centre  for  values  of  x  between  0  and  -,  on  the  left  of  the 

centre  for  values  of  x  between  -  and  -,  and  correspondingly  for  higher 

4          2 

values  of  x. 

Many  subjects  are  able  to  give  to  such  intra  -  cranial  tones  a 
definite  localisation  (e.g.  in  the  pharynx  or  cerebellum)  and  can 
accurately  describe  the  path  of  the  tone  as  it  passes  from  ear 
to  ear. 


EXERCISES  OJST  CHAPTER  XXII 

The  Visual  Perception  of  Size  and  Direction 

PROJECTION  OF  AFTER-IMAGES. 

Exp.  140.  The  subject  projects  the  after-image  of  a  square 
object  on  to  surfaces  at  different  distances  from  the  eye,  and  observes 
the  varying  size  of  the  after-image.  He  also  projects  the  after-image 
of  a  cross  on  to  planes  variously  inclined  with  respect  to  the  eye, 
and  observes  the  varying  distortion  of  the  after-image. 

MEASUREMENT  OF  OPTICAL  ILLUSIONS. 

Exp.  141.  A  subject  and  experimenter,  after  they  have 
familiarised  themselves  with  the  commoner  forms  of  geometric 
optical  illusions,  should  proceed  to  the  quantitative  estimation  of 
one  of  them  after  the  following  model. 


EXPERIMENT  141  409 

The  apparatus  may  consist  of  a  board  covered  with  black  cloth,  on 
the  surface  of  which  appear  two '  white  lines  at  right  angles  to  one 
another.  By  a  simple  contrivance  at  the  back  of  the  board,  the 
lengths  of  these  lines  may  be  easily  varied.  The  experimenter  sets 
the  horizontal  line  at  100  mm.  and  the  vertical  line  at  a  few  mm. 
in  length,  and  gives  the  board  to  the  subject,  who  has  to  prolong  the 
vertical  line  until  it  appears  equal  in  length  to  the  horizontal  line. 
While  he  is  lengthening  the  vertical,  the  subject  must  be  careful  that 
the  board  is  in  a  constant  horizontal  (or  vertical)  position  directly 
below  (or  in  front  of)  him.  By  means  of  compasses,  or  by  applying  a 
scale  to  the  board,  the  experimenter  notes  and  records  the  actual  length 
of  this  vertical  line.  He  then  reduces  the  vertical  to  a  few  mm.  in 
length  and  asks  the  subject  to  repeat  the  estimation.  Ten  such 
values  should  be  obtained,  and  their  mean  and  mean  variation 
determined. 

Ten  estimations  should  then  be  made  by  the  subject,  when  the 
vertical  is  initially  longer  than  the  horizontal  and  has  to  be  shortened 
by  the  subject  until  the  two  lines  appear  equal  in  length. 

The  mean  error  and  variability  for  the  twenty  estimations  can 
then  be  calculated,  and  a  comparison  can  be  made  of  the  error  and 
variability  of  estimation  when  the  vertical  is  presented  to  the  subject 
(i.)  longer  and  (ii.)  shorter  than  the  horizontal  line. 

By  turning  the  board  successively  through  90°,  180,°  and  270°, 
three  further  series  of  observations  should  be  made  in  order  to  com- 
pare the  various  errors  of|  estimation,  according  as  the  vertical  lies 
to  the  right  of,  to  the  left  of,  or  above  or  below  the  horizontal  line. 

Experiments  may  also  be  conducted,  in  which  the  vertical  line 
preserves  a  constant  length  of  100  mm.,  and  the  horizontal  has  to  be 
made  equal  to  it  by  the  subject. 

A  very  simple  apparatus  may  be  also  contrived  for  measuring  the 
Muller-Lyer  illusion ;  the  two  parts  of  the  illusion  being  combined 
as  shown  in  the  upper  portion  of  figure  17  (page  300).  The  right- 
hand  half  of  the  figure,  with  both  pairs  of  end  lines,  is  drawn  on  a 
thin  white  xylonite  surface.  This  part  of  the  apparatus  is  fixed  and 
forms  a  framework,  to  the  left  of  which  a  thin  board  of  the  same 
material  slides  in  and  out,  bearing  the  remainder  of  the  figure.  The 
board  can  be  drawn  out  or  pushed  in  beneath  the  framework,  until 
the  two  sections  of  the  horizontal  line  appear  equal. 


EXPERIMENTAL  PSYCHOLOGY 


EXEECISES  ON  CHAPTER  XXIII 

Time 

REPRODUCTION  OF  INTERVALS. 

Exp.  142.  The  student  should  familiarise  himself  with  the 
apparatus  devised  for  recording  equal  intervals  of  time  on  the  smoked 
surface  of  a  revolving  drum.  An  electrically  vibrating  tuning-fork 
may  be  connected  in  the  same  circuit  with  a  time  marker  (fig.  38). 


FIG.  59.— Clock  Time  Marker. 

The  upper  of  the  two  dials  records  minutes,  the  lower  seconds.  The  clock 
is  started  and  stopped  by  the  lever  H.  Movement  of  the  lever  K 
restores  the  clock  hands  to  starting-point  (60).  The  clock  is  wound 
by  turning  a  nut  on  the  side  opposite  to  that  shown  in  the  figure. 
On  the  same  side  is  a  small  stud  which  changes  the  rate  of  movement 
of  the  recording  lever.  This  can  be  made  to  rise  and  fall  every  second 
or  every  fifth  of  a  second.  Its  excursions  may  be  recorded  directly 
on  a  travelling  smoked  surface,  or  they  may  be  used  to  interrupt 
either  of  two  electric  circuits,  the  terminals  for  which  are  shown  at  M. 

But  it  is  more  convenient  to  use  a  specially  devised  clock  (fig.  59), 
bearing  a  lever  which  records  fifths  of  seconds  or  whole  seconds,  as 
desired. 

The  recording  apparatus  should,  even  for  class  purposes,  be  set  up 
in  a  different  room  from  that  in  which  the  subject  sits  whose  accuracy 
of  time  estimation  is  under  investigation.  The  most  reliable  method 
of  presenting  to  the  subject  any  desired  interval  of  time  is  attained 
by  the  use  of  a  uniformly  rotating  metal  arm  which  during  rotation 
comes  into  contact  with  two  (or  more)  sets  of  terminals  ;  the  result  of 
such  contacts  being  to  close  (or  to  open)  electric  ^rcuits,  and  thereby 
to  produce  two  or  more  (e.g.  telephonic)  sounds  absolutely  alike  in 


EXPERIMENT 


411 


character  and  separated  by  an  interval  dependent  on  the  rate  of 
rotation  of  the  metal  arm,  and  the  distance  between  any  two  sets  of 
the  metal  terminals,  which  can  be  accurately  varied  at  will. 

In  place  of  this  apparatus  (fig.  60)  the  interval  may  be  presented 
to  the  subject  by  two  auditory  signals,  given  from  the  room  in  which 
the  recording  apparatus  stands,  and  transmitted  to  the  subject  by 
means  of  a  telephone  "buzzer."  Or  still  more  simply,  a  second 
individual  sitting  not  too  near  the  subject,  with  a  watch  in  hand, 


.-A 


FIG.  60.— The  Leipzic  "Time  Sense"  Apparatus. 

The  arm  A  is  driven  by  the  cog  wheels  K'  K",  and  these  by  the  wheel 
W,  which  is  connected  with  a  reliable  (clockwork  or  other  form  of) 
motor.  As  it  revolves,  the  arm  touches  a  number  of  contacts,  C', 
C",  C'",  C1T.  A  contact,  drawn  from  two  aspects,  is  separately 
shown  in  a  comer  of  the  figure. 

may  give  the  two  stimuli  by  tapping  twice  on  a  Morse  key,  and  the 
time  estimation  is  made  by  the  subject  tapping  similarly  on  a  second 
key. 

"*  If  Morse  keys  are  used,  they  should  be  so  arranged  that  the  taps 
made  on  each  are  communicated  to  a  single  time  signal  which  is 
brought  to  write  on  the  recording  surface  of  the  drum  directly  above 


412  EXPERIMENTAL  PSYCHOLOGY 

the  movements  of  the  lever  of  the  time  marker.  By  this  arrange- 
ment, both  the  length  of  the  interval  presented  by  the  experimenter 
and  the  estimated  interval  returned  by  the  subject  can  be  measured 
and  compared.  The  experimenter  must  be  careful  to  mark  each  of 
the  experimenter's  intervals  on  the  drum,  so  that  later  he  can  always 
distinguish  them  from  the  subject's  intervals.  The  two  keys  may  be 
used  in  a  variety  of  ways.  The  interval  between  two  taps,  a  and  6, 
having  been  given  by  the  experimenter's  key,  the  subject  may  be 
required  to  make  a  third  tap  c  on  his  key,  when  the  interval  between 
b  and  c  appears  to  him  equal  to  that  between  a  and  6.  Or  the  subject 
may  be  required  to  make  two  taps,  c  and  d,  on  his  own  key,  separated 
by  an  interval  equal  to  that  between  a  and  6,  given  by  the  experi- 
menter. In  this  case,  the  intervals  of  the  experimenter  and  subject 
may  be  recorded  on  separate  time  markers.  Or,  again,  the  experi- 
menter, after  having  given  a  and  6,  himself  gives  the  tap  c  after  a 
definite  fixed  interval  has  elapsed,  the  subject  being  enjoined  to  give 
the  fourth  tap  d  on  his  own  key  when  the  intervals  between  c  and  d 
and  between  a  and  b  appear  to  him  equal. 

By  one  of  these  methods,  a  series  of  experiments  should  be  made 
for  intervals  of  different  length  ;  five  tests,  for  example,  being  made 
for  intervals  lying  between  10  and  12  seconds,  five  for  intervals 
between  5  and  6  seconds,  five  for  intervals  of  about  4  seconds,  five  for 
3,  and  so  on  for  2,  l£,  1,  £  and  J  seconds.  The  sounds  should  be 
given  with  uniform  loudness,  and  between  each  group  of  five  tests 
the  subject  should  carefully  record  the  results  of  introspective 
analysis.  tWhen  the  interval  is  very  small,  the  experimenter  will 
himself  be  unable  to  present  it  exactly,  but  this  is  of  little  moment 
as  the  interval  which  he  presents  will  be  accurately  measurable  on 
the  drum  to  which  his  taps  are  transmitted. 

The  percentage  error,  positive  or  negative,  should  be  calculated  for 
each  estimation,  and  the  nature  and  extent  of  the  error  for  the  differ- 
ent lengths  of  intervals,  together  with  the  position  of  the  indifference 
point,  should  be  investigated.  The  results  may  be  further  treated  in 
their  original  groups  of  five,  with  the  object  of  testing  the  variability 
of  the  error  for  different  lengths  of  interval,  but  for  reliable  results 
a  greater  number  of  data  must  be  obtained,  and  the  order  in  which 
they  are  obtained  must  be  taken  into  consideration. 

COMPARISON  BETWEEN  FILLED  AND  EMPTY  INTERVALS. 

Bxp.  143.  A  series  of  experiments  should  be  devised  and  carried 
out  to  show  the  effect  on  time  estimation  which  occurs  when  the 
interval  between  the  two  taps,  a,  6,  is  filled  with  other  taps,  instead  of 


EXPERIMENTS  144-146  413 

being  silent.  Two  intervals  are  successively  presented,  the  one  tilled, 
the  other  empty,  and  the  subject  has  to  determine  whether  the  inter- 
vals are  equal  or  unequal.  The  time  order  of  presentation  of  the 
intervals  should  be  varied,  and  either  the  limiting  or  the  constant 
method  should  be  used  to  determine  the  amount  of  the  illusion. 

Rhythm 

SUBJECTIVE  ACCENTUATION  IN  KHYTHM. 

Bxp.  144.  The  metronome  is  a  convenient  instrument  for 
observing  the  subjective  accentuation  of  the  simplest  rhythm.  But 
care  must  be  taken  that  no  objective  accentuation  of  its  beats  exists. 

The  experimenter  should  set  the  metronome  at  various  rates  of 
oscillation,  so  that  the  subject  may  appreciate  the  relation  between 
rate  of  rhythm  and  ease  of  subjective  accentuation.  The  subject 
should  observe  and  record  the  varying  affective  values  (pleasant, 
wearisome,  etc.)  of  different  rhythms  and  the  associated  experiences 
which  they  may  revive.  The  experimenter  may  notice  unconscious 
movements  on  the  part  of  the  subject. 

OBJECTIVE  ACCENTUATION. 

Exp.  145.  The  effects  of  varying  the  objective  accentuation  are 
easily  studied  by  enclosing  the  metronome  in  a  box,  the  lid  of  which 
may  be  silently  opened  and  closed  at  any  moment  so  as  to  allow  any 
desired  sound  to  be  intensified  and  so  to  be  accented.  Trochaic  -  u, 
iambic  u-,  dactylic  -ou,  anapaestic  uu-,  and  cretic  -u-  measures 
should  be  studied.  The  effect  of  accenting  the  first  of  every  four, 
five  and  six  beats  should  be  studied  for  different  rates  of  rhythm. 

ACCURACY  OF  EEPRODUCTION  OF  KHYTHM. 

Bxp.  146.  The  accuracy  with  which  the  reproduction  of  a 
given  rate  of  rhythm  can  be  maintained,  may  be  investigated  by 
means  of  the  metronome ;  the  subject  reproducing  the  rhythm  by 
tapping  on  a  Morse  key,  the  movements  of  which  are  transmitted  to 
the  recording  surface  of  a  drum  in  another  room.  The  subject 
begins  to  tap  synchronously  with  the  metronome  sounds,  and  after, 
say,  twenty  sounds  have  been  heard,  the  metronome  is  stopped  while 
the  subject  continues  his  tapping.  The  time  signal  or  clock  (fig.  59) 
records  fifths  of  seconds  on  the  drum  below  the  tracings  made  by  the 
taps  of  the  subject. 

The  relation  of  respiratory  movements  to  rhythmical  action  and 


4H  EXPERIMENTAL  PSYCHOLOGY 

to  the  estimation  of  time  may  be  studied  by  attaching  a  pneumograph 
(fig.  63)  to  the  subject  and  by  connecting  it  with  a  tambour  (fig.  64) 
brought  to  bear  on  the  recording  surface. 


EXEKCISES  ON  CHAPTEK  XXIV 

Attention 

FLUCTUATIONS  OF  ATTENTION. 

Exp.  147.  The  subject  is  seated  in  a  silent  room  or  gallery, — 
or,  better  still,  out  of  doors  on  a  quiet  night.  The  experimenter 
holds  a  watch  opposite  one  ear  of  the  subject,  and,  keeping  it  at  this 
level,  withdraws  it  in  a  straight  line  from  the  subject,  until  the  latter 
can  only  just  hear  the  ticks.  The  watch  may  be  hidden  in  a  cloth 
if  it  tick  too  loudly.  Care  must  be  taken  that  the  alternations  of 
sound  and  silence,  now  experienced  by  the  subject,  are  not  com- 
plicated by  any  objective  variations  in  the  loudness  of  the  ticks.  The 
alternations  may  be  graphically  recorded,  as  in  the  following  experi- 
ment. 

Exp.  148.  When  a  white  disc,  bearing  a  thick  broken  black  line 
on  an  imaginary  radius  (fig.  61),  is  rotated  on  the  colour  wheel,  a  series 
of  grey  bands  is  observed  which  become  increasingly  faint  towards 
the  periphery  of  the  disc.  The  subject,  comfortably  seated,  with  his 
head  supported  by  a  head  rest,  fixates  the  faintest  grey  ring  he  can 
distinguish.  He  observes  the  fluctuations  that  it  undergoes. 

After  a  little  practice,  he  will  find  no  difficulty  in  recording  these 
fluctuations,  by  varying  the  pressure  of  his  finger  on  a  rubber  bulb 
which  is  connected  with  a  recording  tambour  (fig.  64).  The  lever  of 
this  tambour  and  a  time  marker  are  brought  to  bear  on  the  travelling 
smoked  surface  of  a  drum  or  kymograph.  The  recording  instruments 
should  be  placed  at  some  distance  from  the  subject,  to  prevent 
distraction.  The  kymograph  should  rotate  quite  slowly,  say  twice 
in  three  minutes.  The  record  should  be  interrupted  after  a  single 
revolution  of  the  kymograph ;  a  longer  sitting  becomes  unsatis- 
factory, owing  to  inattention. 

The  experiment  may  be  modified  in  various  ways.  Instead  of  a 
rubber  bulb,  which  permits  of  a  continuous  record,  a  reaction  key 
may  be  used.  Instead  of  the  faintest  ring,  a  ring  somewhat  less 
faint  may  be  fixated.  The  background,  instead  of  being  white,  may 
be  black  and  the  line  white.  The  degree  of  attention  given  by  the 


EXPERIMENT  149 


415 


subject  to  the  ring  may  be  experimentally  varied.  The  relation 
of  respiratory  movements  (exp.  151)  to  these  fluctuations  may  be 
graphically  studied. 


FIG.  61. 

It  is  essential  that  the  graphic  records  obtained  by  the  experi- 
menter be  supplemented  by  careful  introspection  on  the  part  of  the 
subject. 

THE  SPAN  OF  APPREHENSION. 

Exp.  149.  The  essentials  of  a  good  tachistoscope  have  been 
already  mentioned  in  the  text.  Several  forms  of  the  instrument 
have  been  devised,  (a)  In  the  fall  tachistoscope,  a  screen  carrying 
a  fixation  mark  is  allowed  to  drop.  During  its  fall,  it  momentarily 
exposes  a  card  on  which  various  objects,  e.g.,  letters  or  figures,  are 
arranged. 

(6)  In  the  rotatory  tachistoscope,  the  subject  looks  down  a 
narrow  vertical  blackened  tube  on  to  the  periphery  of  a  large 
horizontally  rotating  white  disc,  which  is  driven  by  a  very  steady 
motor.  The  disc  has  a  sector  cut  out  from  its  margin.  The  open 
sector  allows  the  subject  to  see  a  card  of  letters,  etc.,  placed  below 
the  disc.  The  rate  of  rotation  of  the  disc,  the  area  of  the  sector,  and 
hence  the  time  of  exposure,  can  be  varied  at  will.  Fixation  is 
secured  by  a  preliminary  trial  in  which  an  easy  letter  is  shown  in 
place  of  the  card  of  objects. 


416 


EXPERIMENTAL  PSYCHOLOGY 


(c)  111  the  pendulum  tachistoscope  (fig.  62),  an  oblong  screen  C, 
provided  with  a  central  aperture,  is  fixed  to  the  free  end  of  a  pendu- 
lum. The  pendulum  is  held  up  by  an  electro- magnet,  and  released 
at  the  desired  moment.  During  its  swing  the  screen  momentarily 
exposes  the  objects  which  lie  behind  it.  At  the  end  of  its  swing  it  is 
caught  by  the  catch  D.  An  optical  arrangement  can  be  fitted  to  the 
pendulum  tachistoscope,  by  means  of  which  the  images  B  of  stencilled 
lines,  letters,  or  figures,  placed  behind  the  screen,  can  (during  the 
momentarily  favourable  position  of  the  aperture)  be  thrown  forward 
by  aid  of  the  condenser  A  and  the  lens  E  on  to  a  plate  of  ground  glass 


FIG.  62.— Hales's  Tachistoscope. 


at  F,  which  carries  a  fixation  mark  fixated  by  the  subject.  The 
subject's  head  is  supported  by  a  head  rest. 

Whatever  instrument  be  employed,  the  student  should  proceed  in 
the  first  place  to  determine  the  maximum  span  of  apprehension,  keep- 
ing to  a  uniform  time  of  exposure  and  varying  only  the  number  (not 
the  nature)  of  the  objects  exposed.  Dots,  differing  in  number  and  in 
arrangement  within  a  given  area,  form  a  suitable  material. 

A  warning  signal  is  given  at  a  fixed  time,  say  a  second,  before  the 
object  is  exposed.  After  the  object  has  been  once  exposed,  the  subject 
describes  what  he  has  seen.  The  experimenter  may  use  the  limiting 
method,  proceeding  from  a  single  dot  to  two,  three,  four  or  more  dots 
until  the  limit  of  the  range  of  attention  is  passed.  Or  he  may  use  the 


EXPERIMENTS  150,  151  417 

constant  method,  varying  the  order  of  exhibition  irregularly,  and 
observing  the  proportion  of  right  and  wrong  answers  for  different 
numbers  of  dots. 

Bxp.  150.  The  student  should  next  proceed  to  investigate  the 
influence  of  meaning  and  previous  familiarity  upon  the  range  of 
attention,  by  exposing  letters  grouped  in  senseless  and  in  sensible 
combinations. 

Careful  introspective  records  should  always  be  made  by  the  subject 
and  subsequently  correlated  with  the  results  obtained  by  the  experi- 
menter. 


EXEECISES  ON  CHAPTEB  XXV 

Feeling 

N.B.  The  brief  description  of  the  few  following  instruments  is 
only  intended  to  give  the  student  an  idea  of  the  general  principles 
on  which  they  are  constructed.  The  details  of  the  instruments  are 
certain  to  differ  in  different  laboratories.  It  must  be  left  for  the 
teacher  to  indicate,  and  for  the  student  to  learn  by  experience,  the 
exact  methods  of  manipulation.  The  tracings,  made  by  the  instru- 
ments on  a  recording  surface,  need  to  be  accompanied  by  two  other 
tracings.  Of  these  the  one  records  time  intervals  of  a  second,  while 
the  other  indicates  the  moments  of  applying  and  discontinuing  the 
stimulus  which  is  to  cause  a  change  of  feeling. 

THE  PNEUMOGRAPH. 

Bxp.  151.  This  instrument  records  the  rate  and  extent  of  respiratory 
movements.  In  the  simple  form  figured  (fig.  63),  it  consists  of  a  metal 


FIG.  63.  FIG.  64. 

cylinder  closed  at  the  two  ends  by  rubber  sheeting.     A  hook  is  attached 
to  the  centre  of  each  piece  of  rubber,  the  two  hooks  being  connected 
by  a  piece  of  tape  which  passes  tightly  round  the  chest  of  the  subject. 
27 


4i 8  EXPERIMENTAL  PSYCHOLOGY 

The  fluctuations  of  air  pressure  within  the  pneumograph,  thus  pro- 
duced by  respiratory  movements,  are  communicated  to  a  recording 
tambour  (fig.  64)  by  means  of  a  side  opening  in  the  metal  wall  of  the 
cylinder  to  which  a  piece  of  rubber  tubing  is  attached. 

In  a  complete  investigation  of  respiratory  movements,  two  pneumo- 
graphs  should  be  employed,  since  variations  in  the  thoracic  movements 
are  by  no  means  always  accompanied  by  like  variations  in  the  abdom- 
inal movements  of  respiration. 

THE  SPHYGMOGRAPH. 

Exp.  152.  This  sensitive  instrument  (of  which  there  are  very 
different  forms),  when  applied  to  the  pulse,  e.g.  at  the  wrist,  responds 
to  minute  changes  in  arterial  pressure.  These  changes,  constituting 
the  pulse,  are  communicated  by  levers  to  a  recording  surface.  Varia- 
tions in  the  form  of  the  pulse  curve  or  in  the  frequency  of  the  pulse 
are  indicated  on  the  sphygmographic  record. 

THE  PLETHYSMOGRAPH. 

Exp.  153.  This  consists  of  a  closed  chamber  in  which  part  of 
the  body,  usually  the  forearm,  comfortably  rests.  The  chamber  is 
connected  with  a  distant  tambour,  so  that  any  variations  of  pressure 
due  to  increased  or  decreased  volume  of  the  arm  are  transmitted  to 
a  recording  lever.  In  some  patterns  of  the  instrument,  the  chamber 
enclosing  the  arm  contains  air,  but  more  usually  the  air  is  replaced 
by  lukewarm  water. 

THE  AUTOMATOGRAPH. 

Exp.  154.  In  investigating  the  effect  of  pleasant  and  unpleasant 
stimuli  upon  the  contraction  of  skeletal  muscle,  it  is  of  course  essential 
that  the  subject  should  so  far  as  possible  remain  in  ignorance  of  the 
effects  which  are  expected  to  occur,  and  that  the  effects  which  have 
occurred  should  be  concealed  from  him  until  all  the  desired  experi- 
ments are  completed. 

The  automatograph  is  a  convenient  instrument  for  recording 
involuntary  movements  of  the  arm  (fig.  65).  It  is  a  freely  swinging 
"  planchette  "  A  B,  in  which  the  arm  comfortably  rests.  The  slightest 
to  and  fro,  or  lateral,  movement  of  the  apparatus  is  communicated  by 
a  glass  style  C  to  an  underlying  piece  of  smoked  paper.  The  subject's 
eyes  are  closed  throughout  the  experiment. 

The  movements  of  the  automatograph  should  first  be  studied  when 
the  subject  preserves  a  dreamy  attitude  of  complete  indifference. 


EXPERIMENT  155  419 

After  this  has  been  done,  the  experimenter  observes  the  effect  of  intro- 
ducing a  pleasant  or  unpleasant  stimulus.  Odours  (e.g.  asafoetida, 
castor  oil,  musk,  jockey  club)  are  the  easiest  to  use ;  they  can  be 
silently  brought  beneath  the  subject's  nostrils.  Their  effect  on 
involuntary  movement  is  to  be  carefully  noted  by  the  experimenter 


FIG.  65. 

and  correlated   with  the  introspective  record  subsequently  obtained 
from  the  subject. 

It  is  important  that  a  given  odour  should  not  be  brought  into  the 
room  until  it  is  needed,  and  that  the  subject  should  be  given  ample 
rest  between  the  applications  of  different  stimuli. 

THE  KECORDING  DYNAMOMETER. 

Bxp.  155.  For  rough  work  the  instrument  figured  below  (fig.  66) 
will  suffice.  Its  purpose  has  been  sufficiently  indicated  on  page  334. 
A  very  slowly  rotating  drum  should  be  used  to  record  the  movements 
of  the  lever  L.  The  subject  is  enjoined  to  make  a  maximal  contrac- 
tion at  F  and  to  concentrate  his  attention  on  maintaining  this  degree 
of  contraction  for  about  a  minute.  The  eyes  are  closed  as  before.  A 
time  signal  records  seconds  upon  the  drum.  A  preliminary  tracing  is 


420 


EXPERIMENTAL  PSYCHOLOGY 


taken  with  the  subject  in  an  indifferent  state.     The  subject  should 
carefully  analyse  the  state  of  his  conciousness  throughout  the  experi- 


FIG.  66.    (After  Titchener 


ment.  After  adequate  pauses,  tracings  are  taken  in  which  the  state 
of  indifference  is  replaced  by  one  of  pleasure  or  displeasure,  owing 
to  the  exhibition  of  an  appropriate  stimulus  at  recorded  times. 


INDEX 


[The  numbers  refer  to  the  pages,] 


Absolute  impression,   267-272,  314, 

398. 

threshold  (see  Threshold). 
Acmfesthesia,  13. 
Acoumeter,  Politzer's,  248,  394. 
Acutesthesia,  13. 

Acuity,  sensory,  243-253,  393-398. 
algesic,  251. 
auditory,  247,  248. 
Mbliogi'aphy,  253. 
gustatory,  249. 
motor  (kinresthetic),  251. 
olfactory,  248. 
tactual,  249,  250. 
thermal,  250,  251. 
visual,  244-246. 

Adaptation,  auditory,  252,  253,  291. 
labyrinthine,  253. 
in  mental  work,  195. 
muscular,  253  (see  also  Attunement). 
olfactory,  252,  253. 
sensory,  252. 
tactual,  18. 
thermal,  16,  343. 
in  determining  threshold,  207. 
visual,  79,  96, 252,  253,  256 ;  to  twi- 
light, 87-89,  97,  101,  363,  364. 
jEsthesiometer,  190  (see  also  Thresh- 
old, spatial). 
.Esthetics,  328-331. 
After- sensations  (see  under  Auditory, 

Visual,  etc.,  Sensations). 
Age  influence,  on  size-weight  illusion, 
220  ;  on  learning,  176,  181  ;  on 
range   of  audible  pitch,  37  ;  on 
reactions,  137  ;  on  area  of  sensi- 
bility to  taste,  110. 
Algesic  acuity  (see  under  Pain). 


Allochiria,  233. 
Anosmia,  114. 

Apprehension,  span  of,  323,  324,  415. 
Appunn's  Tonmesser,  346. 
Aristotle's  experiment,  237,  390,  391. 
Association  of  colours  with  names,  49. 
in  feeling,  330. 
in  harmony,  49,  50,  58. 
mediate,  169-171. 
in  memory,  144-182. 
and  motor  attunemant,  226. 
related.  166,  167. 
retro-active  and  remote,  164-166. 
unconscious,  167,  168. 
Associations,  age,  influence  of,  173- 
175  ;  classification  of,  151,  152, 
381. 

times,  141,  381. 
wrong,  by  active  substitution,  170  ; 

passive,  171. 
summation     effects     in     rational 

learning,  178. 
Atropin,    and   fluctuation   of  visual 

sensations,  321,  322. 
and  visual  perception  of  size,  293, 

294. 

Attention,  317-326,  414-417. 
MbliograiJJiy,  325. 
in  binocular  rivalry,  280. 
in    "complication"    experiments, 

318. 

distraction  of,  322. 
effect    of,    on   apparent    order    of 
presentations,    317  ;     on    their 
duration,  316 ;  on  rhythm,  315. 
fluctuations  of,  320,  414. 
in  learning,  177. 
measurement  of,  325. 


421 


422 


INDEX 


Attention — 

presentation  order,  317. 

in  reactions,  133,  134,  136,  138. 

span  of  apprehension,  323,  324,  415. 
Attunement  (see  under  Motor  Sensa- 
tions). 

Audiles,  147. 

Auditory  sensations,  11-62,  344-351  ; 
primary,  120-122. 

acuity,  247,  248,  394. 

after-sensations,  33,  347. 

adaptation,  252,  397,  398. 

beats,  37-41,  348,  349 ;  binaural, 
40,  53. 

beat  tones,  46,  54. 

bibliography,  41,  62. 

binaural  experience,  286-292  (sec 
also  under  Binaural). 

combination  tones,  42-46,  350. 

conduction  of  sounds  to  inner  ear, 
20  ;  from  ear  to  ear,  21,  40,  53, 
288  ;  to  one  ear  only,  21. 

consonance  and  dissonance,  29, 351 ; 
determinants  of,  48,  57  ;  theories 
of,  48,  56-58. 

difference,  tones  (see  above  Com- 
bination tones). 

diplacusis,  binaural,  52  ;  uniaural, 
52. 

discrimination  of,  37,  120,  256, 
257,  266. 

fatigue  of,  252,  253. 

fluctuation  in,  320,  321,  414. 

Fourier's  theorem,  22,  24. 

hearing,  theories  of  (Ebbinghaus), 
56;  (Ewald),  59-61;  (Helm- 
holtz),  24, 51-59, 61 ;  (Meyer),  61 . 

imagery,  144-147. 

intensity,  change  of,  confused  with 
change  of  pitch,  32,  35,  291  ; 
physical  basis  of,  20  ;  of  simul- 
taneous tones,  35,  347  ;  threshold 
of,  32,  247,  248,  394. 

interruption  tones,  46,  47,  54. 

intertones,  37-41,  45,  53,  57,  61, 
350. 

middle  ear,  function,  20,  21,  25. 

noises,  22,  27,  55,  345  ;  localisation 

of,  290  (see  also  below  Tones), 
overtones,    30,    31,    44    (see    also 

Timbre). 

phase  influence,  on  localisation  of 
sound,  287,  407,  408 ;  on  tonal 
experience,  53,  54,  61. 


Auditory  sensations — 

pitch,  absolute  determination  of, 
association  with  names,  49;  by 
birds,  50  ;  by  children,  50  ; 
methods  of,  49  ;  difference  of, 
37,  50,  55,  120,  248,  257; 
physical  basis  of,  20 ;  range  of 
audible,  36,  348. 

resonance,  22-25,  43,  51-59,  345. 

rhythm,  315,  316. 

summation  tones  (see  aborc  Com- 
bination tones). 

timbre,  change  of,  confused  with 
change  of  pitch,  31,  40  ;  import- 
ant in  localisation  of  sound,  289- 
291  ;  meaning  of,  28  ;  physical 
basis  of,  21  ;  relation  of  over- 
tones to,  30-32,  346,  347. 

tone  character,  34,  35. 

tone  gaps,  52. 

tones,  nomenclature,  28-30  ;  purity 
of,  39,  57,  58,  121  ;  relation  to 
noise,  26-28,  36,  55 ;  relation 
to  one  another,  47-49,  351. 

variation  tones,  46. 

Weber's  law,  256,  257. 
Auerbach  and  Miiller-Lyer's  illusion, 

300. 
Automatograph,  418. 

Beats  (see  under  Auditory  Sensations). 
Benham's  top,  363. 
Binaural  experience,   286-292,   405- 
408    (see    also  Auditory  Sensa- 
tions). 

adaptation  in  localisation,  291. 
intensity  differences,  287,  288,  405. 
labyrinthine      differences,       286 ; 
theories    of    (Miinsterberg    and 
Preyer),  287. 

phase  differences,   287,  288,  407  ; 
theories     of     (Lord     Rayleigh, 
Myers  and  Wilson),  288. 
pitch  differences,  348. 
tactual  differences,  286. 
temporal  differences,  287. 
timbre  differences,  289,  290,  405. 
Binocular    experience,    274-285   (see 

also  Visual  Sensations), 
brightness,  281-284,  404. 
combination,  274,  400,  403,  404. 
contrast,  281,  403,  404. 
corresponding  retinal  points,  274- 
277,  400-402. 


INDEX 


423 


Binocular  experience — 
covering  or  identical  points,  275. 
cyclopean  eye,  276,  401. 
diplopia,  274,  275,  401,  402. 
disparation,  retinal,  274-277. 
flicker,  281-283. 
movements,  277,  284,  404. 
rivalry,  280,  403. 
Blind  spot,  276,  397. 
Boredom,  in  relation  to  fatigue,  194. 
Brentano,  on  the  visual  estimation  of 

angles,  298. 
Breuer   on    labyrinthine    sensations, 

65. 

Brewster's  prism  stereoscope,  401. 
Brightness  (see  under  Visual  Sensa- 
tions and  Binocular  Experience). 
Brown,      Crum,      on      labyrinthine 
sensations,  65. 

Camera  acustica  (Ewald's),  60. 
Charpentier's  bands,  £6,  361. 
Children,  immediate  memory  in,  180. 
range  of  audible  pitch  in,  37. 
reaction  times  in,  137. 
sensibility  to  taste  in,  110. 
size- weight  illusion  in,  220. 
visual  imagery  in,  147. 
Chronoscope  (Hipp's),  219,  371-379, 

382. 

Ccsnaesthesia,  16. 
Clock  time  marker,  410,  413. 
Coherence  of  stimuli,  272,  273. 
Conn's  test  for  visual  acuity,  393, 

394. 

Cold  spots,  11-16,  18,  340. 
Colour-blindness,  83-85,  89,  99,  360  ; 
theories  of  (see  under  Colour  Sensa- 
tions). 

Colour  sensations   (see  also  Visual), 
adaptation  of,  79,  96,  252,  396. 
after-image,   coloured   waning   of, 
363  ;  negative,  79-81,  90,  93-95, 
98,  357,  358;  positive,  89,  90, 
95,  101,  363. 
anomalous,  77,  85. 
black,  nature  of,  103-105. 
Charpentier's  bands,  86,  361. 
contrast,  binocular,  281,  403,  404  ; 
simultaneous,  81,  94,  100,  102, 
358-360  ;    successive   (see   above 
under  After-images), 
flicker,  85,  86, 103,  361  ;  binocular, 
281-283. 


Colour  sensations — 

induction,  simultaneous  and  suc- 
cessive, 81,  95,  100,  364. 
at    periphery    of   retina,    79,    97, 

357. 
Purkinje's  phenomenon,  87,  88,  89, 

97,  362,  363. 

rivalry,  binocular,  200,  403. 
rod  vision,  87,  88,  101. 
theories  of,  88,  92-107  ;  Donders's, 
106;   Helmholtz's,  92,    93,    98- 
101,  103;   Hering's,  93-105;  v. 
Kries's,   106 ;  McDougall's,  99  ; 
Maxwell's,   92;    Miiller's,    105; 
Rollet's,  99  ;  Young's,  92. 
Colour  wheel,  414. 
Colours,  complementary,  78-80. 
fundamental,  97. 
primary,  81. 
Colourless  sensations  (see  also  Visual), 

characters  of,  76,  355,  356. 
in  colour  blindness,  83. 
conditions  of,  77-79. 
contrast  (see  under  Colour  Sensa- 
tions). 

theories  of,  93-107. 
Combination  tones  (see  under  Auditory 

Sensations). 
Comparison  (see  also  under  Difference, 

Weber's  law), 
imagery  in,  148-151. 
without  imagery,  150. 
Compasses  (Weber's),  190,   389   (see 

also  Threshold,  spatial). 
Consonance,   theories   of   (see  under 

Auditory  Sensations). 
Contrast  (see  under  Colour  Sensations, 
Visual  Perception,  and  Psycho - 
physical  Methods). 
Correlation,  129-131,  368-370. 
Corresponding  retinal   points,    274- 

277,  400-402. 

Curve,  frequency,  126,  192,  212. 
normal  ("probability"),  125-127, 

129. 

Curve  of  work  (see  Work). 
Cutaneous    sensations,    11-19,    340- 

344. 

acuity  (or  sensibility),  tactual, 
249,  250  ;  thermal,  250,  251  ; 
algesic,  251. 

adaptation,  16-18,  252,  343. 
bibliography,  19. 
fatigue,  252\ 


424 


INDEX 


Cutaneous  sensations — 
local  signature,  231-240,  242. 
and    motor,    distinction  between 

68,  69,  351. 
tactual  stimuli  and  time  estimation, 

310,  311. 
Weber's  law,  255. 
Cyclopean  eye,  276,  401,  402. 

Dark  adaptation,  87-89,  97,  101,  363, 

364. 
Deaf  mutes,  labyrinthine  defects  in, 

67. 

Depth,  perception  of,  274-280. 
Difference,  experience  of,  201,  255- 
273,   398-400   (see  also  Psycho- 
physical  Methods), 
absolute  impression,  267-272,  314, 

398. 

bibliography,  273. 
coherence  in,  272. 
Fechner's  law,  258-263. ' 
practice  effects,  266. 
side  comparisons,  272. 
time  error,  266. 
Weber's  law,  205,  255-273,  398; 

basis,  264. 
Difference  tones  (see  under  Auditory 

Sensations). 

Differences,  individual,  in  sensory 
acuity,  245-247,  249;  in 
aesthetic  preferences,  330,  331  ; 
in  Aristotle's  experiment,  391  ; 
in  imagery,  145-150  ;  their  im- 
portance for  psychology,  10  ;  in 
learning,  167,  175,  179-182;  in 
muscular  efficiency,  189 ;  in 
range  of  audible  pitch,  52  ;  in 
retaining  practice  effects,  197  ; 
in  reaction,  136-138  ;  in  colour 
relief,  281  ;  in  sensibility,  110, 
111,  115  ;  in  estimating  size  of 
heavenly  bodies  at  horizon,  305  ; 
in  spatial  threshold,  390 ;  in 
time  error,  266;  in  "type  of 
judgment,"  271 ;  in  colour  vision, 
85. 

Differential  threshold  (see  Threshold). 
Diplopia,  229,  275,  401. 

and  disparation,  275,  276. 
Direction    of   sounds,    286-291    (see 

also  Binaural  Experience). 
Disparation,   crossed  and   uncrossed 
retinal,  274-277,  400-402, 


Distance  of  sounds,  291,  292. 
Distraction,  322,  325. 
Donders's  theory  of  vision,  106. 
Drainage  theory  of  nervous  energy, 

100,  102,  284,  320. 
Drugs,  effect  of,  12,  84,  88,  111,  153, 

181,  233,  293,  294,    309,    321, 

322. 
Dynamometer,  187,  383,  419. 

Ebbinghaus's  explanation  of  Weber's 

law,  264,  265. 
theory  of  hearing,  56. 
Efficiency,  mental  and  muscular  (see 

Work), 
visual,  246. 

Effort,  "sense"  of,  227-229. 
Einthoven's  theory  of  the   Miiller- 

Lyer  illusion,  299. 
Emotion  (see  Feeling). 
"  Empathy,"  factor  in  determination 

of  feeling  (Lipps),  331. 
Energy,  specific,  of  sensations,  117- 

122. 

Epicritic  sensibility,  13,  14. 
Episcotister,  362. 

Equality,  experiences  of  (see  Psycho- 
physical  Methods). 
Equation,  personal  (see  Reactions). 
Ergograph  (Krapelin's),  383. 
Ergography.  184-187,  190. 
Error,  probable,  127,  128,  367,  368  ; 
space,  203,  204,  208,  270 ;  time, 
203,  204,  208,  266. 
Ewald's  theory  of  hearing,  59-61. 
Expectation,  effect  of,  in  determina- 
tion of  threshold,  207  ;  on  sensi- 
bility, 244  ;  in  time  estimation, 
313. 
Experience,    binaural,    286-292   (see 

also  under  Binaural). 
binocular,  274-285  (see  also  under 

Binocular). 

of  identity  and  difference,  201, 
255-273  (see  also  under  Differ- 
ence). 

Experiment,  psychological,  aims  of,  5. 
abstraction  in,  7. 
behaviour  of  subject  in,  4. 
introspection  in,  3. 
simplicity  in,  153,  198-200. 
statistical  methods  in,  123-131. 
£ye  movements  (see  Orbital  Move- 
ments). 


INDEX 


425 


Fatigue,  in  association  experiments 
153. 

auditory,  252,  253. 

in  memory,  144,  155,  181. 

mental,  192-198  ;  measurement  of, 
196,  197. 

muscular,  185-188. 

olfactory,  252,  253,  366. 

in  reactions,  135. 

of  temperature  spots,  341. 

visual,  93,  98,  252. 
Fechner's  (and  Helmholtz's)  theory  ol 
complementary  after-sensations, 
93. 

time  error,  204,  266. 

verification  of  Weber's  law,  258. 

colours,  86,  361,  363. 

law,  258-263 ;  critical  examina- 
tion of,  260. 

paradox,  283,  284. 

side  window  experiment,  404. 
Feeling,  327-337,  417-420. 

bibliography,  337. 

determinants  of,  327,  330,  331. 

and  expression,  332-336  ;  analysis 
of  relation  (Royce,  Titchener, 
Wundt,  and  James-Lange),  335, 
336. 

movements,  organic,  332  ;  skeletal, 

333  ;  during  anaesthesia,  336. 
Fourier's  theorem,  22,  24. 
Foveal  and  extra-foveal  vision,  79, 
87-90,  101,  238,  246,  357,  361, 


Galtou's  whistle,  36,  348. 
Gauss's  law  of  error,  125,  211. 
Giddiness,  66-68. 

Gustatory  sensations,  108-112,  364- 
365. 

acuity  (or  sensibility),  249. 

bibliography,  116. 

chemistry   of   tasting   substances, 
109,  110. 

in  children,  110. 

compensation,  111,  112,  365. 

contrast,  111,  112,  365. 

drugs,  action  of,  111. 

primary,  120. 

region,  110. 

rivalry,  111,  112. 

tastes,  classification  of,  108,  109 ; 
olfactory,  110,  11 5, 


Head    (and    others)    on    cutaneous 

sensibility,  12-14. 
Heat  spots,  11-16,  18,  340,  341. 
v.  Helmholtz's  explanation  of  visual 
after-sensations    of    movement, 
241 ;  theory  of  hearing,  24, 51-59, 
61,  121. 

theory  of  colour  vision,  92,  93; 
difficulties  of,  98-101,  103, 
105. 

Hering,  temperature  adaptation,  17, 
18,  344  ;  nature  of  black,  49- 
96  ;  simultaneous  and  successive 
induction,  81,  95,  364 ;  local 
signature,  238,  239. 

Bering's  apparatus  for  binocular 
colour  combination,  403  ;  de- 
monstration of  binocular  com- 
bination and  uniocular  contrast, 
404  ;  view  of  retinal  disparation, 
277  ;  fall  experiment,  402 ; 
geometrical  illusion,  302,  303  ; 
explanation  of  certain  visual 
illusions,  296,  297  ;  theory  of 
colour  vision,  93-105. 

Hipp's  chronoscope,  219,  371-379, 
382. 

Holmgren's  wools,  360. 

Horopter,  276. 

Hypersesthesia,  12. 

Hypnosis,  expression  of  feeling  dur- 
ing, 336. 

Identity  and  difference,  experiences 
of,     255-273     (see    also    under 
Difference). 
Illusions — 

Aristotle's,  237,  390,  391. 

of  colour,  77-81  (see  also  Visual 
Sensations). 

of  distance,  276-279,  291,  292,  295, 
400,  402,  403. 

Fechner's  paradox,  283,  284,  404. 

geometrical  optical,  295-305  ; 
angles,  wrong  estimation  of, 
297,  298  ;  confluence  aud  con- 
trast, 298-301  ;  Bering's  figure, 
303  ;  Miiller-Lyer's  figure,  299- 
301,  304,  305,  409  ;  perspective 
figures,  303,  304,  322  ;  Poggen- 
dorff's  figure,  301  ;  filled  and 
empty  space,  296,  297  :  verticals 
and  horizontals,  295,  '296,  408, 
409  ;  Conner's  figure,  301,  302. 


426 


INDEX 


Illusions — 

heavenly  bodies,  of  size  of.  305. 

306. 

of  localisation,  236-240,  276,  286- 
291,  390-392,  401,  402,  405-408. 
of  movement,  65,  66,  70,  71,  228, 
229,  234,  240-242,  390,  392,  393. 
of  pitch,  31,  32,  348. 
of  position,  73. 
of  relief,  279-281,  401, 
of  rhythm,  315,  316,  413,  414. 
of  rotation,  65,  66. 
of  temperature,  16-18,  343,  344. 
of  time,  308-315,  317-319,  410-413. 
of  touch,  231-234,  389-391. 
of  weight,  219-224,  344,  388,  389. 
Imagery  in   memory,    144-151    (see 

Memory). 
Impression,  absolute,  267-272,   314, 

398. 

Improvability  in  mental  work,  197. 
Incitement,  195. 

Individual  differences,  CM  Differences. 
Induction,    simultaneous    and    suc- 
cessive, 81,  95,  364. 
Inference,  unconscious,  in  size-weight 
illusion,      220 ;     in     geometric 
illusions,  304. 

Inhibition  in  mental  work,  193. 
in  muscular  work,  186. 
retro-active,  in  memory,  163. 
Interference   tubes,    for   removal    of 

overtones,  44,  45. 

Interpretation,  influence  of,  on  visual 
acuity,  246  ;  on  spatial  thresh- 
old, 390. 

Interruption  tones  (see  under  Audi- 
tory Sensations). 

Intervals,  musical,  47-49,  56,  351. 
Introspection    in    experiment,    2-5, 
152,    216,    232,   267,   286,  325, 
328. 

limitations  of,  4. 
objections  to,  3. 
value  of,  3,  5,  198,  339. 
v.  re£ro-spection,  3. 
Itching,  sensation  of,  9. 

James-Lange  theory  of  emotion,  336. 

Kinsesthesia,    definition    of,    63    (see 

also  Motor  Sensations). 
Kbnig's  views  of  combination  tones, 

45,  46,  54. 


Kriipeliu's  calculation  test,  386. 

ergograph,  383. 

work  curve,  195. 
v.  Kries's  theory  of  rod  vision,  89, 101. 

theory  of  colour  vision,  106. 

Laboratory  exercises,  339-420. 
Labyrinthine  sensations,   63-68,   74, 

351. 

adaptation,  253. 
bibliography,  75. 
and  binaural  experiences,  286  ; 

(Preyer  &  Miinsterberg),  287. 
end  organs,  function  of,  64  ;  Mach- 

Breuer-Brown  theory,  65. 
giddiness,  66-68. 
and  motor,    comparison  between, 

63,  74. 

rotation,  effect  of,  66-68. 
saccule,  function  of,  64,  68. 
utricle,  function  of,  64,  68. 
Learning,  rational,  compared  to  me- 
chanical, 179  (see  also  Memory). 
Leipzic  time  sense  apparatus,  411. 
Limen  (see  Threshold). 
Lipps's    insistence    on    "empathy," 

302,  331. 

Listing's  law,  284,  404. 
Local  signature,  231-242. 

Aristotle's  experiment,    237,    390, 

391. 

cutaneous,  231-238,  390-392. 
Bering's  theory,  238,  239. 
Lotze's  theory,  238,  239. 
motor  (kinsesthetic)  sensations  in, 

237-240. 

retinal,  231,  238-242,  392. 
threshold,  spatial,  231-237,   389- 

393. 

Localisation  (see  under  Local  Signa- 
ture, Sound), 
Locomotor  ataxia,  kinsesthetic  sense 

in,  70,  219. 

Lotze's    theory   of   local    signature, 
238,  239. 

McDougall's  explanation  of  Fechner's 

paradox,  284. 
of  graded  contrast,  102. 
of  simultaneous  contrast,  99. 
Mach's  theory  of   the    function    of 

the  auricles,  290. 

Mach-Breuer- Brown  theory  of  laby- 
rinthine sensations,  65, 


INDEX 


427 


Martin  and  Miiller,  absolute  impres- 
sion, investigations,  270. 
Maxwell's  theory   of  colour  vision, 

92,  93. 
Mean,  median  (see  under  Statistical 

Methods). 
Meissner's  corpuscles  and  touch  spots, 

11. 
Memory,  144-182,  380-383  (see  also 

under  Association), 
age  influence,  180. 
bibliography,  161,  182. 
forgetting,  rate  of,  162,  178. 
group  reproduction,  168. 
the    "identical     series"    method, 

156. 
images,     144  -  151,    380  ;     after  - 

images,  148. 
immediate,  179,  180. 
individual  differences,  181. 
inhibition,  retro-active,  163. 
initial  reproduction,  168. 
the  "learning"  method,  153-155. 
learning,  economical  methods  of, 

171-177,  182. 

length  of  series,  influence  of,  157-9. 
mediate,  180. 
perseverance  of  experiences,    144, 

179-182. 
position  of  syllables,  influence  of, 

159,  160. 
practice,   influence    of,    177,    178, 

180,  181. 
the    "prompting"    method,    154, 

159,  160. 

reading,  speed  of,  178. 

recognition,   distinct   from   repro- 
duction, 181. 

repetitions,   distribution   of,  171- 

173. 
rhythm  and  accent,   influence  of, 

160,  168,  177. 

the  "saving"  method,  153-156, 
381. 

the  "scoring"  method,  154,  160, 
173,  382. 

the  "selection  "  method,  156. 

time  influence,  149,  150,  174,  175. 
Meniere's  disease,  character  of,  67. 
Mental  excitement,  effect  on  muscu- 
lar work,  188,  189. 

fatigue,  effect  on  muscular  work, 
188,  189  (see  under  Mental 
Work). 


Mental   work,    183,    189-200,    385, 
386. 

adaptation,  195, 

sesthesiometric  test,  190. 

bibliography,  200. 

calculation  test,  191,  386. 

combination. test,  190,  385. 

continuous  tests,  189,  191-198. 

curve,  192-194. 

ergograph  test,  190. 

fatigability,  196. 

fatigue,  192-198. 

improvability,  197. 

incitement,  195. 

interpolation  tests,  190. 

laboratory  work,  criticism  of,  198. 

learning  test,  191. 

letter-erasing  test,  190. 

and  muscular,  compared,  183. 

pause  effects,  195-198. 

practice  effects,  192. 

retentiveness  of  improvement,  197. 

spatial  threshold  as  test,  190,  233, 
385. 

spurts,  194  ;  initial  and  end,  194. 
Menthol,  effect  on  heat  spots,  12. 
Metamorphopsia,  239. 
Methods,    of    learning    (see    under 

Memory) ;  the  psycho  physical  (sec 

Psychophysical) ;     statistical    (see 

Statistical). 

Meyer's  theory  of  hearing,  61. 
Microphone,  40. 

Micropsia,    at  fixation    point,    293 ; 
beyond  fixation  point,  294,  295. 

in  presbyopia,  294. 

in  retinitis,  295. 

Mode  (see  under  Statistical  Methods). 
Motiles,  147. 

Motor  anaesthesia,  68,  218,  219. 
Motor  sensations,  63,  68-75,  351,  352. 

acuity,  251. 

ataxia,  effect  of,  68,  219. 

attunement,  221-227. 

bibliography,  75. 

central  "sense  "  of  effort,  227,  228, 
229. 

of  cramp,  71. 

end  organs,   69  ;   effect  of  faradic 
current,  69. 

fatigue,  71. 

illusions,  66,  70,  352. 
and  labyrinthine,  resemblance  be* 
tvveen,  63,  74. 


428 


INDEX 


Motor  sensations — 
local  signature  attributed  to,  237- 

240. 

memory  images,  145-147. 
in  orbital  movements,  295,  296. 
of  pressure,  71. 
of  strain,  71. 
Movement,  of  eyes  (see  Orbital) ;  in 
feeling    (see    Feeling) ;    in    re- 
actions, 136  ;  tactual  perception 
of,  234,  390  ;  visual  perception 
of,  240-242,  392,  393. 
Miiller,    G.    E.,    theory    of    colour 

vision,  105. 
and   Martin,    absolute   impression 

investigations,  267-272. 
Miiller's,  Johannes,  theory  of  specific 

nervous  energy,  118,  120,  121. 
MiilJer-Lyer's  illusion,  299-301,  304, 

305,  409. 

Auerbach's  explanation,  300. 
effect  of  practice  on,  305. 
Einthoven's  explanation,  299. 
eye  movements  in  traversing,  304. 
in  primitive  people,  301. 
Miinsterberg's    theory,   of   auditory 

localisation,  287. 
of  relation  between  time  estimation 

and  respiration,  313. 
Muscular  and  mental  activity,  inter- 
relation of,  183. 

sensations  (see  Motor  Sensations). 
Muscular  work,  183-189,  383-385. 
aifection,  effect  of,  188. 
bibliography,  200. 
influence  of  fatigue,  185-189;  peri- 
pheral and  central  factors,  185. 
impulses,       afferent,       inhibitory 
effects    of,    186,    187  ;    sensory 
effect  of,  188. 
interest,  effect  of,  188. 
loads,  constant  and  variable,  186. 
practice,  188,  189,  385. 
Musical  intervals,  47-49,  56,  351. 
Myers    and     Wilson,     influence    of 
binaural  phase  differences,  288. 

Nervous  energy,  specific,  118-122. 
theory  of  drainage,  100,  102,  284, 

320. 

and  Weber's  law,  257. 
Noise,  26-28,  36,  55,  345. 
Normal  curve,  in  statistical  methods, 
125. 


Nicotin,  producing  colour-blindness, 

84. 

Olfactometer  (Zwaardemaker's),  248, 

249,  366. 
Olfactory  sensations,   108,    112-116, 

365,  366. 

acuity,  248,  249,  395. 
adaptation,  253. 
anosmia,  114. 
bibliography  i  116. 
classification,  113,  114,  116. 
compensation,  115,  366. 
conditions,  112. 
defects,  114,  115. 
fatigue,  115,  253. 
in  relation  to  gustatory  sensations, 

108,  110,  115. 
hallucinations,  115. 
primary,  120. 
rivalry,  115,  366. 
Weber's  law,  257. 
Opium,  effect  on  experience  of  time, 

309. 
Orbital  movements,  70,  277-279,  284, 

295,  296,  298,  302-306,  404,  405. 

Pain,  12-16,  342,  343.       . 

acuity  of,  251. 

adaptation,  252. 

effect  on  pulse  and  respiration,  333. 

"referred,"  15. 

specific  nature  of,  15,  117. 

spots,  14,  15,  342. 
Painless  areas,  14,  15,  343. 
Papillse,    of    tongue,    reactions     of, 

110,  111,  364,  365. 
Perimeter  for  experiments,  in  vision, 

357  ;  on  sound,  405. 
Perseverance,  in  memory,  144, 179-82. 
Personal  equation  (see  Reactions). 
Perspective,    illusions  of,   303,    304, 
322 ;    influence    of,     on    visual 
estimation  of  size,  295,  303. 
Pitch  (see  under  Auditory  Sensations 

and  Binaural  Experience). 
Plethysmograph,  332,  418. 
Pneumograph,  332,  322,  417. 
^oggendorffs  figure,  301. 
Jolitzer's  acoumeter,  248,  394. 
3osition,  awareness  of,  68,  70,  73. 
Dractice  at  geometric  illusions,  304. 

at  mental  work,  192-198. 

at  muscular  work,  188,  189,  385. 

and  sensory  acuity,  244. 


INDEX 


429 


Presentations,    apparent    order    of, 

317-320. 

Pressure  (see  Touch). 
Preyer's  theory  of  auditory  localisa- 
tion, 287. 

Psychology,  aims  of,  1,  5,  10,  153, 
203,  204. 

physiological,    in   relation  to    ex- 
perimental, 8. 

statistical  methods  in,  123. 
Psycho-physical  methods,  201-217. 

bibliography,  217. 

the  constant  method,  202,  210-217, 
388,  389,  398,  399. 

contrast  effects,  213,  272. 

method  of  equal-appearing  inter- 
vals, 215,  399,  400. 

the  limiting  method,  202,  205-209, 
215,  216,  388. 

method   of  mean  error,    202-205, 
215,  216,  217,  387. 

reversals,  212,  213. 

side  comparisons,  213,  272. 

space  and   time  errors  (sec  under 
Error). 

subject's  attitude,  216. 

uses  of,  201. 

Purkinje's  phenomenon,  87,  88,  362, 
363. 

Hering's  explanation  of,  97. 

rod  vision  as  cause  of,  88,  89,  101. 

Quincke's  tubes,  350. 

Race  influence,  in  reactions,  137 ;  in 

geometric  illusions,  301. 
Range,  semi -interquartile  (see  under 

Statistical  Methods). 
Rational  learning,  178-180. 
Rayleigh,    Lord,  on  binaural  phase 

differences,  288. 

Reaction  times,  132-143,  370-379. 
Reactions,  age  influence  in,  137. 

analysis   of,    mathematical,    141  ; 
psychological,  135. 

associative,  141,  379. 

'bibliography ,  143. 

choice,  139. 

chronoscope  (Hipp's),  371-379. 

composite,  132,  138-141,  379. 

definition  of,  132. 

discriminative,  138. 

fatigue  effects  in,  135. 

individual  variations  in,  136. 

movements  in,  136. 


Reactions — 

muscular,  133. 

natural,  136. 

personal  equation,  137. 

practice  effects  in,  135. 

physiological  aspects  of,  142. 

race  influence  in,  137. 

recognitive,  138. 

reduced,  or  central,  132,  133,  135. 

sensorial,  133. 

simple,  132-138,  378. 
Repetitions,  distribution  of,  in  learn- 
ing, 171-173  ;  in  effecting  motor 
attunement,  227. 

Reproduction  (see  under  Memory). 
Resistance,  experience  of,  71,  352. 
Resonance,  of  auricles  (Mach),  290  ; 

principle  of,  22-25,  345  ;  in  theory 

of  hearing,  51-56. 

Resonators,  use  of,  24,  27,  30,  43,  46. 
Rest,  effect  of,  on  mental  work,  195-8. 
Retentiveness,    of    improvement    in 
mental  work,  197. 

of  memory,  173,  179,  181  (see  also 

Memory). 

Retinitis,  producing  distorted  vision, 
239. 

micropsia  in,  295. 
Reversals,    in    the    psycho-physical 

methods,  212,  213. 
Rhythm,  308,  315-316,  413. 

accentuation  in,  315. 

bibliography,  316. 

maintenance  of,  316. 

in  memory,  160,  168,  177. 

reproduction  of,  316. 

respiratory,   and   time    estimation 

(Miinsterberg),  313. 
Rivers,    "W.    H.    R.,    on    micropsia 

beyond  the  fixation  point,  295. 
Rod  vision,  88-90,  101. 
Rollet's  theory  of  vision,  99. 
Royce's  analysis  of  feelings,  335. 
Rutherford's  theory  of  hearing,  59. 

Santonin,  producing  colour-blindness, 

84. 

Schultze's  theory  of  rod  vision,  89. 
Scotoma,  central,  84. 
Sensation   (see  also  under  Auditory, 
Cutaneous,  Gustatory,  etc.,  Sen- 
sibility). 

acuity  of  (see  Acuity), 
algesic,  251  (sec  also  Pain). 


430 


INDEX 


Sensation — 
analysis  of,  8. 
articular,  and  awareness  of  position, 

69,  70. 

auditory,  20-62. 
autokinetic,  240. 
complex,  8. 
cutaneous,  11-19. 
discrimination  as  factor  in,  243. 
of  effort,  229. 
gustatory,  108-112. 
immeasurability  of,  263. 
kinaesthetic,  63,  68-75. 
labyrinthine,  63-68,  74. 
modal  differences  of,  119. 
motor,  63,  68-75. 
muscular,  69. 
olfactory,  108,  112-116. 
primary,  120-122. 
qualitative  differences  of,  119. 
specific  energy  of,  117-122. 
statesthetic,  73. 
temperature,  11,  12-18. 
visceral,  11-19. 
visual,  76-107. 
Sensibility,    243-247,    393-398    (see 

also  Acuity). 

adaptation  and,  16, 79, 96,  252, 253. 
bibliography,  253. 
conditions  determining,  243,  244. 
cutaneous,  two  systems  of,  12-14. 
fatigue  and,  16,  244,  252,  253,  341. 
influence  of  interpretation,  244, 246. 
physiological    and     psychological 

aspects  of,  244. 
visual.  246. 

Sex    influence,    in   colour-blindness, 
83  ;  in  "type  of  judgment,"  271. 
Signature,  local  (see  Local). 
Silence,  aural  conditions  in,  34. 
Size- weight  illusion,  220,  221,  388. 
Skin  (see  Cutaneous  Sensations). 
Smell  (see  Olfactory  Sensations). 
Snellen's  illiterate  visual  test-types, 

393,  394. 
Sound,  conduction  of,  20,  21,  40,  53, 

288,  344. 
direction  of,    286,    291    (see    also 

Binaural  Experience), 
distance  of,  291,  292. 
pictures  (Ewald's),  60. 
waves,  20,  21,  22,  26,  30,  38,  53, 
54,    288,    289;    sine,    21,    26; 
effects  of  single,  26,  27. 


Space     error,     in     psycho  -  physical 

methods,  203,  204,  208,  270. 
Spatial  threshold  (see  Threshold). 
Sphygmograph,  332,  418. 
Sphygmomanometer,  332. 
Spurts  in  work,  194. 
Standard  deviation,   124,   126,   127, 

360-367. 

"  Statsesthetic  "  sensations,  73. 
Statistical  methods,    123-131,   367- 

370. 

bibliography,  131. 
correlation,  129-131,  368-370. 
curve,    frequency,   126,  192,  212  ; 
normal    ("probability"),    125- 
127,  129. 

Gauss's  law  of  error,  125,  211. 
mean    (average),    124,    127,    367, 

368. 

mean  variation,  124,  367,  368. 
median,  128,  367. 
mode,  124,  126. 
probable    error,     127,     128,     367, 

368. 

semi-interquartile  range,  129,  367. 
significant  differences,  125-128. 
standard  deviation,  124,  126,  127, 

367-370. 

variation,  co-efficient  of,  124. 
Stereoscope,  274,  277,  401,  403. 
mirror  (Wheatstone's),  401. 
prism  (Brewster's),  401. 
Stern's  tone  variators,  349,  350. 
Stimuli,    adequate   and   inadequate, 

117. 

coherence  of,  272,  273. 
effective  and  ineffective,  117. 
Stumpfs  theory  of  consonance  and 

dissonance,  58. 

Summation  tones  (see  under  Audi- 
tory Sensations). 

Sun  and  moon,  estimation  of  size  of, 
305. 

Tachistoscopc,  415-417. 
Talbot-Plateau  law,  86,  282,  361. 
Taste  (see  Gustatory  Sensations). 
Temperature,  acuity,  250. 

adaptation,  16-18  ;    (Hering),  17, 
18  ;  (Weber),  16. 

after-sensations,  344. 

cutaneous  sensibility  to,  12,  13,  14, 
16-18,  340-341,  343-344. 

and  weight  illusion,  344. 


INDEX 


Threshold,  absolute,  32,  69,  201,  202, 

205-209,  240,  244-252,  320-322. 

differential,  37,  201,  202,  205-217, 

255,  273,  325. 
spatial,    210,   211,   231-236,    325, 

385,  389. 

Tickling,  sensation  of,  18. 
Time,  308-315,  410-413. 
bibliography,  316. 
influence    in  binaural   experience, 

287. 

displacements,  318. 
error  in  psycho-physical  methods, 

203,  204,  208,  266. 
filled  and   empty  intervals,   310- 

313,  410,  412. 
fusion  of  experiences  in  estimation 

of,  308,  309. 
respiration,  affecting  estimation  of, 

311,  313  ;  (Miinsterberg),  313. 
sense  apparatus  (Leipzic),  411. 
specious   or  sensory  present,   308, 

309. 

surprise,  expectation,  313. 
Titchener's  analysis  of  feelings,   335. 
Tonmesser,  346. 
Touch,  acuity,  249,  250. 
adaptation,  18,  252. 
light  and  heavy,  12,  13. 
spots,  11,  15,  341,  342. 
local  sign  of,  232. 
Traube-Hering  waves,  321. 

Variation  tones  (sec  under  Auditory 

Sensations). 

Viscera,  sensations  of,  14,  15. 
Visiles,  147. 

Visual  perception  of  movement,  240- 
242,  392,  393  ;  of  size  and  direc- 
tion, 293-307,  408. 
after-images,    projection    of,    295, 

408. 

angles,       estimation      of,      297  ; 
(Brentano),  298  ;  (Hering),  297  ; 
(Wundt),  298. 
bibliography,  306. 
contrast  and  confluence  in,    298, 

301,  304. 
geometric  illusions,  295-305,  322, 

408. 

metamorphopsia,  239. 
micropsia  at  fixation  point,  293  ; 
beyond    fixation  point,  294  ;  in 
presbyopia,  294  ;  inretinitis,  295. 


Visual  perception  of  movement — 

Muller-Lyer's  figure,  299,  304. 

perspective,  influence  of,  295,  303. 

Poggendorfi's  figure,  301. 

space,  filled  and  empty,  296,  297. 

Zollner's  figure,  301. 
Visual   sensations,   76-107   (see  also 
under     Binocular     Experience, 
Colour-Blindness,     Colour    and 
Colourless  Sensations). 

acuity,  244-246,  393. 

adaptation,  79,  96,  252. 

after-sensations  of  movement,  241, 
242. 

attention  to,  317-319  ;  fluctuations 
in,  320,  321,  414,  415. 

bibliography,  91,  107. 

binocular  experience,  274-285  (see 
also  under  Binocular). 

brightness,  definition  of,  77  ;  de- 
termination of,  86,  356,  361, 
362  ;  effect  of  dark  adaptation, 
87,  88  ;  theories  of,  88,  97,  284  ; 
in  binocular  vision,  281-284, 
404. 

characters  of,  76,  77. 

of  colour  (see  under  Colour). 

efficiency,  246,  355-364. 

fatigue,  93,  98,  252. 

induction,  simultaneous  and  suc- 
cessive, 81,  95,  100,  364. 

local  signature,  231,  238-242. 

primary,  120. 

rivalry,  binocular,  of,  280,  403. 

sensibility,  246. 

Talbot- Plateau  law,  86,  282,  361. 

theories  of,  88,  92-107. 

and  time  illusions,  310. 

and  Weber's  law,  256. 

Ward,  Prof.  J.  (footnote],  331. 
Weber's  compasses   (sesthesiorneter), 

190,  389-391. 
Weber's  law,  255-258,  264-273,  398- 

400. 
absolute  impression,  267-272,  314, 

398,  399. 
basis,  264,  265. 
coherence  of  stimuli,  272. 
differential     threshold     of     time 

intervals,  273. 
Ebbinghaus's  interpretation,  264, 

265. 
Fechner's  verification,  258-264. 


432 


INDEX 


Weber's  law- 
irradiation,  theory  of,  265. 

limits  of,  256,  257. 

Wundt's  interpretation,  264. 
Weight,  218-230,  388,  389. 

absolute  impression,  267-272,  398, 
399. 

adaptation,  253. 

anaesthesia,   effects   of  motor  and 
tactual,  218. 

attunement,  motor,  221-227. 

bibliography,  230. 

effort,  sense  of,  227-230. 

influence  of  size,  220,  221,  388,  389. 

speed  of  lifting,  219-221. 

influence  of  temperature,  344. 

Weber's  law,  255,  256. 
Wheatstone's  mirror  stereoscope,  401 . 
Whistle,  Galton's,  36,  348. 
Wilson  and  Myers,   binaural  phase 
differences,  288. 


Work  curve,  mental,  192. 

Krapelin's  analysis  of,  195  (see  also 
Mental     Work    and     Muscular 
Work). 
Wundt's  analysis  of  feelings,  335. 

explanation  of  wrong  estimation  of 
angles,  298. 

insistence  on  tactual  basis  of  audi- 
tory localisation,  286. 

interpretation    of    Weber's    law, 
264. 

Young's  theory  of  colour  vision,  92, 
93,  98-101,  103. 

Zero,  physiological,  17. 
Zollner's  figure,  301. 
Zwaardemaker's    olfactometer,    248, 

249. 
olfactory  sensations,  classification 

of,  114  ;  theory  of,  116. 


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